I actually like Micro$ofts online Live One Care. It's free and cleans out your computer, registry, scans for viruses and spy/add/malware, defrags the hard drive, and a few other things. It actually does a really good job considering it's Micro$oft. I run it's full service scan on my computers once a month. It takes a few hours to run, but it really does a nice job (And it's actually killed viruses and addware on my system that none of the other big name virus/spyware scanners were able to get rid of.)
http://onecare.live.com/site/en-US/d....htm?s_cid=sah