I had some adware that was killing my hard drive. I had the latest norton and it did not catch it. I trashed it and downloaded AVG's freeware. Got my stuff back running normal. I plan on upgrading to the full version on payday.
I second that. I used SUPERAntiSpyware to rid a neighbor's system of some nasty stuff.
I've used many of the other apps recommended in this thread thus far, and have not been as impressed with them as I was with SUPERAntiSpyware. The price for the personal version is right too - free -- http://www.superantispyware.com
I've always had good luck with superantispyware. Click preferences, then scanning control, and then make sure, close browsers before scanning and terminate memory threats before quarantining are checked. Now go ahead and do a complete scan. These options are for superantispyware.
All these FREE programs are FREE for a reason. Spend the $50 for Zone Alarm Security Suite and save yourself the hassle. The activation code is good for 3 computers and it doesn't take much RAM to run it. Plus it replaces the Windows firewall too. I haven't had any problems with any spyware of viruses slowing down 3 computers in 4 years.