Bet you didn't knew such free scans exists.
MalwareBytes Anti Spyware
This above is a small MS Windows trial application which can scan for all known and recent malwares on your computer. Saved my Windows PCs and several other friends/colleagues too!
Rid your PC of worms, spywares, trojans..
Especially if you have got the xp antivirus spyware which mimicks a Blue Screen of Death! aka BSOD. The trial version just disables the auto scheduled scanning and auto updating, both which can be manually done without much effort.
Trend Micro Housecall Online Virus Scan
The next one above is a free online scanner of your PC for all known viruses.
Any platform, Windows, Linux, Mac etc etc.
I usually run the 2 apps above over the weekend or on a boring monday..
Kills all viruses and spyware on your PC left over by infected mails and evil websites.
Free Windows Vista/XP/2000/NT registry scan to optimize your system to top speed