Presumably for windows OS?
Spybot search and destroy is a good one.
Adaware (both free and for profit) used to be really good, now it's a bit slow and crappy but it's a good idea to run both.
Careful if you go googling for some too as you'll some across some nasties in disguise.
Incidentally you may already be doing this but there are other forms of protection too such as a non directx browser (Opera, Firefox, Konqueror (for *nix)) and firefox coupled with adblock and noscript is a pretty formiddable setup.