Have you tried Windows Live OneCare Safety Scanner?
I have. It really is free and it really does work. The catch? After it scans and removes the bad stuff from your computer, it gives you the choice to either try the 90 day free trial or to skip it. That's it.
Windows Live OneCare safety scanner: Free online tool for PC health and safety
Get a free PC safety scan
Windows Live OneCare safety scanner is a free service designed to help ensure the health of your PC.
Check for and remove viruses
Get rid of junk on your hard disk
Improve your PC's performanceNew: Windows Vista users, try the beta edition of the scanner, just for Windows Vista.
Fix specific PC issues
Use the full service scan to check everything. To help fix particular problems on your PC, turn to the individual scanners below.
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Full Service Scan
Learn How to Protect Your PC
To help protect your computer against a wide variety of security threats, see Protect Your PC.