Spybot
Really... it works, especially in concert with Norton AV/Personal Firewall.
I got great results running Spybot with AVG freeware antivirus and the free version of ZoneAlarm, too - but when I spent the money for the Norton bundle (to get the diagnostic/repair Utilities), and uninstalled AVG and ZA, I kept Spybot. I run it about once a week, and whenever I've been surfing "questionable" sites.