MWAV eScanAV Anti-Virus Toolkit is a free portable utility which can scan your PC to detect and remove any viruses, spyware or other threats which might be present.
The program is considerably larger than most of the competition, at a chunky 130MB. But it has more features, too. It's easy to customise exactly what you'd like to scan, for instance: you might run a quick check of RAM and services, check just the startup folders and Registry, or you can run a check on a particular drive or folder.
You also get a useful "View Network Activity" option, which shows you all the open network connections on your system, ideal for catching malware as it "phones home".
Clear log files then provide details on any malware the program might have uncovered. And a Virus List option shows you exactly what MWAV is able to detect (though it's a very long list, so don't click the Generate button unless you're happy to wait for a while).
There are so many of these portable virus scanners that it's hard to get excited about another, but MWAV's unusual degree of configurability, and extras like the network traffic monitor, mean it could be worth a look.