Heard about this one on Tech TV. Seems to work pretty good so far.
It's called "Spybot - search & destroy". It hunts out adware and other software like trojan horses.
You can get it at: security.kolla.de
Die Gator Die!!!
Might as well point out another application in this genre. I used Lavasoft's Adaware (http://www.lavasoftusa.com/software/adaware/) for a while before I found Spybot S&D. Adaware seems more "idiot proof," you just have to push the "start" button and you're on your way. I do like S&D better, though, since it does offer you a new %systemroot%\system32\drivers\etc\HOSTS file, which resolves some commonly known ad servers to localhost instead of their intended destination, effectively blocking many ads.
--Beanie