Malwarebytes is one of the most popular free on-demand virus scanners. In my experience, it has found more malicious files than any other virus scanner I've used.
You can perform a threat scan to detect infections in locations malware normally hides, such as in the memory, registry, and file system. A custom scan can be launched as well, which scans any or all hard drives and folders with the option to include memory objects, startup items, the registry, archives, and rootkits.
Malwareybytes can also detect potentially unwanted programs (PUP) and modifications (PUM), start scans from the context menu in Windows Explorer, and exclude files/folders from scans.
SUPERAntiSpyware Free Edition is a free on-demand virus scanner that detects spyware in the registry and file system as well as those running in memory.
Some advanced settings are available like scanning just cookies and including/excluding particular folders from a scan. You can also use SUPERAntiSpyware Free Edition to scan only the files running in memory, called a Critical Point Scan, which is useful if you think a piece of malware is currently running. This also gives you a quick view of potentially unwanted programs installed on your PC, and a dead simple method to remove them.
There's also a section of the program to show the applications that start up with Windows, as well as SUPERDelete, which lets you remove files that won't properly delete, and the ability to only scan files that were modified within a certain period of time.
Other useful tools and features are included in this program, like a startup manager, scanning only recently modified files, ignoring large files, ignoring System Restore files, and scanning inside ZIP archives.
Something I find very annoying with SUPERAntiSpyware Free Edition is that some of the options are only available in the Professional version, but it's not easily decipherable as to which features are or aren't. Clicking these areas will show an ad to download the full version, which can get annoying.
Note: Be sure to choose the red Free Edition Download button on the download page to avoid getting a trial copy of the professional version of SUPERAntiSpyware.
Emsisoft Emergency Kit provides on-demand virus scanning via 3 built-in tools that run from a single program suite that can be launched and even updated from a portable location.
Emergency Kit Scanner is one of the included tools that scans for viruses, Trojans, spyware, rootkids, adware, PUPs, worms, etc., in a similar fashion as the programs in this list. Particular file types can be excluded from a scan, the inside of archives can be scanned, and it has the option to check for malicious items in the memory. This should be your first option when using Emsisoft Emergency Kit.
The Commandline Scanner is just like the previous scanner but doesn't use a graphical interface. Instead, it runs entirely from a command line interface and should be used mainly for advanced users.
The final tool, to be used with great caution, is BlitzBlank. This program schedules registry entries, files, drivers, and/or folders to be removed when the computer reboots, which is perfect if the malware is preventing itself from being shutdown under normal conditions. This tool is very powerful, so it can be destructive if used inappropriately.
Stinger is a free, portable on-demand virus scanner from the same developers as the McAfee Antivirus software. Its scan targets include running processes, boot sectors, registry keys, and rootkits.
You can create a custom scan with Stinger to just search in particular directories and/or hard drives, or a complete scan that searches through everything. You can see an entire list of threats it can find from the included Threat List.
Stinger can be setup to repair, report, rename, or remove malware once detected. If you know the hash of files you don't want scanned, Stinger lets you build a list of them so the scanner skips over them.
The download size is around 15 MB and doesn't require installation, enabling Stinger to run from a flash drive or external hard drive.
ClamWin Free Antivirus is a free on-demand virus scanner that can be installed to Windows or launched from a removable USB drive.
ClamWin Free Antivirus can scan just the system memory to find viruses that are currently running, or scan the entire file system for malicious items. You can choose to scan a whole drive for viruses or just a particular file or folder.
Scheduled scans are supported in ClamWin Free Antivirus, as well as Window Shell integration, email alerts, file exclusions, and Microsoft Outlook email scanning.
The installable download for ClamWin Free Antivirus is around 100 MB.
Note: The portable ClamWin Free Antivirus install is available through the ClamWin User Guides page.
Dr.Web CureIt! is the exact same software as what's in the bootable Dr.Web LiveDisk program, but works within Windows instead of from a disc. It's a completely portable virus scanner, making it a great way to provide free on-demand virus scanning capabilities without having to install something.
You can scan boot sectors, the memory, the Windows folder, temporary files, system restore points, rootkits, or custom files and folders.
While Dr.Web CureIt! is portable, meaning you don't have to install anything to your computer, the download size is relatively large at around 180 MB.
Note: The Dr.Web CureIt! setup file may be named something different each time it's downloaded to help circumvent being blocked from a virus, so ensure you know where you're saving it.
EMCO Malware Destroyer is a free program for Windows that provides quick and on-demand virus scanning.
The following threats are scanned for with EMCO Malware Destroyer: Trojans, Adware, viruses, worms, dialers, spyware, and suspicious programs.
Thousands of virus signatures are scanned against the PC in question, and they're always being updated. This means EMCO Malware Destroyer is a good choice against both new and old threats.
While testing the program, a full system scan took less than 30 seconds to complete, making it one of the faster on-demand virus scanners from this list.
eScanAV Anti-Virus Toolkit is a completely free portable virus scanner for Windows.
It checks for spyware, adware, keyloggers, and similar potentially malicious items running in the system memory. eScanAV Anti-Virus Toolkit can also scan an individual folder or whole disk drive.
You have the option to just scan without repairing any files, as well as the option to include scanning archive files.
You can also use eScanAV Anti-Virus Toolkit to view and terminate connections to the Internet, which is useful if you suspect a malicious program is accessing the Internet.
Updates can be performed because MWAV installs itself to a temporary location while you use the program. Though this is also a downfall because it means you have to run the setup file again if you wish to use the program in the future, given that all the files were unloaded to a seemingly undisclosed location on your computer.
Something else a bit bothersome about this program is that it displays an advertisement every time you close it.
The file is around 150 MB, so downloading MWAV may take some time.
Microsoft Safety Scanner is a free on-demand portable scanner from Microsoft that checks for and removes spyware and similar malicious files.
You can run a quick scan to check only critical areas where viruses may be resting, a full scan to check every file and folder on your PC, or a custom scan to couple a quick scan with a scan of any other folder of your choosing.
The installer file for Microsoft Safety Scanner is 125 MB and runs as a portable application. See Am I Running a 32-bit or 64-bit Version of Windows? to know which download link to choose.
Note: After 10 days, you're required to download the setup file again if you wish to make another scan. This is to ensure you have the latest definitions to fight malware.
Norton Power Eraser is a portable on-demand free virus scanner that's just 3 MB in size.
The featured virus scanning function in Norton Power Eraser is activated with the main button called Scan for Risks on the first window of the program. This requires a reboot as it scans for rootkits.
Choose Advanced Options for other scanning methods. You can scan a file or folder with the Reputation Scan, which performs a cloud lookup on the data. This requires an active Internet connection so Norton Power Eraser can upload the necessary information to the Internet.
Another method is the regular System Scan option, which scans the current Windows operating system for viruses. The third and final option is similar to the second and is only effective if you're running more than one Windows operating system. If that's the case, you can use the Multi-Boot Scan method to select the other OS that should be scanned.
Sophos Virus Removal Tool is an on-demand free virus scanning tool for Windows that scans for viruses in the file system and memory.
Every time the program is started, it automatically begins an update to be sure its definitions are current, which is nice.
You won't find custom settings in Sophos Virus Removal Tool like file/folder exclusions, scheduled scanning, or context menu integration like similar products offer. However, the scan is thorough and the program lets you remove any viruses it finds when it's finished, which is still helpful.
The program has to be installed to be used, and is around 130 MB in size, so the download may take some time to complete.
Note: You need to enter your name and email address to reveal the download link for Sophos Virus Removal Tool.
Trend Micro HouseCall is a portable, lightweight on-demand virus scanner for Windows. It finds and eliminates Trojans, viruses, worms, and spyware.
The program automatically updates when you open it so as to stay current with recent releases and definition updates.
You can run a quick scan, full scan, or custom scan to select specific folders and drives.
Trend Micro HouseCall is completely portable and just 2 MB in size, making it ideal to place on a portable drive.
Note: See Am I Running a 32-bit or 64-bit Version of Windows? to see which link to choose on the download page.
Kaspersky has a free on-demand virus scanner called Kaspersky Virus Removal Tool.
The program can scan system memory, hidden startup objects, boot sectors, archives, whole hard drives, and specific folders.
There are also settings that can make the tool scan certain file types only and skip files that are taking too long to scan.
Kaspersky Virus Removal Tool is around 100 MB and can be used as a portable antivirus program that doesn't require installation.
Norman Malware Cleaner is an on-demand virus scanner for Windows that's portable and free to use.
Normal Malware Cleaner can scan inside archives, search for fake antivirus software, identify rootkits, and detect malicious items running in system memory. There's also a Forensics section to scan running files to find anything that looks suspicious and could possibly be malicious.
Options are available to exclude files and folders from a scan and to delete or restore quarantined items that Normal Malware Cleaner has found.
The download size of Norman Malware Cleaner is around 300 MB. This is quite a large sized file, but that's because the program isn't installed to your computer, but instead ran as a portable program.
Comodo Cleaning Essentials is a portable virus and rootkit scanner. You can run a smart scan to scan critical areas of the computer or do a full or custom scan.
If you choose a custom scan, you can use Comodo Cleaning Essentials to scan in the memory, critical areas and boot sector, hidden registry objects and services, and/or hidden files and folders.
You can even choose to disable the virus scan so you can check for malicious items in the memory only, for example, instead of searching through the whole computer.
Advanced settings are available such as scanning in archives, excluding large files from a scan, scanning for suspicious MBR modifications, and enabling the creation of restore points before running a scan.
Comodo Cleaning Essentials also includes Autorun Analyzer, which is used for viewing all the programs, drivers, scheduled tasks, and other things that launch when Windows starts. Also available is KillSwitch, which lets you shut down running programs, processes, and network connections.
Note: There's a 32-bit and 64-bit version of the program on the download page that, when extracted, uses from 40 MB to 50 MB of space.
If you can't get into Windows to start a scan, you can even run this program using Comodo Rescue Disk.
Rizone Virus Cleaner is a portable on-demand free virus scanner that finds viruses in running processes and the file system.
The program runs without installation, so it's perfect for a USB device. Just select the drive that should be scanned and then click Go! to begin.
Rizone Virus Cleaner doesn't have many of the advanced options you find in some of the other on-demand cleaners in this list, but it's simple to use and doesn't require installation.
The developers of Panda Free Antivirus also have an on-demand tool for Windows that can scan for viruses, called Panda Cloud Cleaner.
This download is just 30 MB in size and scans in the registry, system memory, and other common locations where malware could be hiding. You can also scan individual files and folders, shut down all running processes, unlock files, and submit suspicious files to Panda for analysis.
If a PC is infected in a way that it's being blocked from running EXE program files, you can choose to download the portable Panda Cloud Cleaner (also on the download page) and run the included VBS file to bypass the executable restriction. This is very handy if you need to scan for viruses but can't open any program.
If neither the installable nor the portable program are helping, the same developers also have Panda Cloud Cleaner available from a bootable program, called Rescue ISO, that you can run before Windows even starts.
Zillya! Scanner is a free on-demand portable virus scanner for Windows from the same developers as Zillya! Antivirus and Zillya! LiveCD (a bootable AV program).
You can run a quick scan to find viruses in sensitive folders, a full scan to scan every area of a drive, or a custom scan to choose particular folders, files, and drives to scan.
When running a custom scan, you have the option to just check potentially dangerous file types for viruses so Zillya! Scanner doesn't have to scan every single file.
Zillya! Scanner requires 140 MB of free disk space.
Multi Virus Cleaner 2011 is a very simple and easy to use on-demand virus scanner.
Only a few settings are found in this program, which makes it much less confusing than most other virus scanner software.
One option is whether to prompt you when a malicious item is found or to disinfect it automatically.
The other settings are simply where you want to scan, like a quick scan against system files, a full scan for all areas of the local drive, a deep scan, or a custom scan.
You can also scan the memory to check all the running processes for worms, Trojans, and spyware.
F-Secure Online Scanner is another free on-demand portable virus scanner. This program doesn't scan every location on a PC, but instead looks through folders where viruses are normally found.
There are absolutely zero custom settings in F-Secure Online Scanner. If it finds a virus, it's removed without question and doesn't prompt you to decide to keep it.
Ashampoo Virus Quickscan is just that - a quick virus scanner. It's completely portable with a small size and scans awfully fast, completing in just a few minutes when I tested it.
The whole computer isn't scanned with this program, but instead just critical areas.
VIPRE Rescue is a portable, on-demand free virus scanner. It doesn't have a user interface but, instead, runs through Command Prompt.
After downloading VIPRE Rescue (which is over 300 MB), the installer will ask you to extract the files to a folder, which requires over 500 MB of free space. The default folder chosen to save the files to is "C:\VIPRERESCUE\" but you can change it to whatever you want.
It will run the program automatically after the extraction, but if not, open the extracted files and run VipreRescueScanner.exe. A scan will immediately start and will automatically remove any viruses it finds.
Note: I recommend only using this program if every other scanner in this list fails to install due to a virus infection. There aren't advanced options and it's too easy to accidentally stop the scan by exiting the Command Prompt window.
List of Free On-Demand Virus Scanners
Free Standalone Virus Scanning Software Tools
An on-demand virus scanner is an antivirus program that you run only when you want to run it - it doesn't protect you from viruses and other malware all the time like those that I've written about in my Best Free Antivirus Programs list.
So what good is an on-demand virus scanner? These types of antimalware tools are great when you think you might have some kind of virus or other malware but the antivirus program you're using now hasn't caught it. Sometimes, a really nasty piece of malware will prevent your installed antivirus program from running properly, in which case an on-demand virus scanner can come in handy.
Here's a complete list of the very best free on-demand virus scanners available. Many of these programs even come in a portable version, meaning no installation required!
Note: In many ways, online virus scanners are also a kind of on-demand scanner. See my Free Online Virus Scanners list if you're interested in going that route instead.