I ripped this right from TechRepublic blog, here. So good!
1. ClamWin Portable
ClamWin Portable is very much like its big brother: It’s free, open source, and does a great job of disinfecting machines. ClamWin has a very high detection rate, has frequently updated definitions, and has an easy to use graphical interface. The only caveat to using ClamWin is that it does not offer a real-time scanner – which is not an issue for a portable version. This is my go-to portable virus scanning software.
2. Sophos Anti Rootkit Portable
Sophos Anti Rootkit Portable is one of those tools you hope you never have to use; but you know, at some point, you will. Sophos is remarkably adept at locating root kits – especially for a portable app. Sophos: scans, detects and removes rootkits, is 100% free, supports Windows XP, Vista and 7, and works alongside your existing antivirus. I have found Sophos reliable enough to use even while the PC being scanned is in use.
3. Emsisoft Free Emergency Toolkit
Emsisoft Free Emergency Toolkit is a powerful malware removal tool that can scan for, and remove, over six million dangers to your PC. Emsisoft Free Emergency Toolkit has both a GUI and a command line version, so you can scan your machine even if there are problems with the GUI. With this toolkit, you not only get the malware scanner, you also get HiJackFree and BlitzBlank as well. Emsisoft offers the free download, or you can purchase a pre-compiled USB stick.
4. Vipre Rescue
Vipre Rescue is that tool you use when your machine is severely infected. Vipre is run in safe mode and does not depend upon a GUI tool for use. You double-click the executable and a command window opens with the scanner running (and running at blazing speeds). If you already use the full version of Vipre, you can still run this tool should your machine become so infected, Vipre will not run.
5. Spybot Search and Destroy Portable
Spybot Search and Destroy Portable is the portable version of the massively popular full Spybot Search and Destroy. This antimalware tool does a great job of finding and removing malicious software – all from your flash drive. Spybot has a unique feature that will help you backup your registry before you begin the scan. Should Spybot fubar your PCs registry, you have a backup to restore to – safe and sound.