If anything use McAfee 8 Enterprise that I get free from school. It does a decent job.
I usually recommend AVG, Spybot, AdAware, and ClamAV.
Recently, I heard about Blink (
http://www.eeye.com/html/products/bl...nal/index.html) which is free for a year, and is supposed to do a decent job. I haven't tried it yet though.
Edit: I forgot about NOD32. I've heard alot of good things about it, but never used it. Michelle's comparitives seem to support it too.
For real security though I reccomend a router (built in firewall) and some common sense. Don't go surfing to sketchy sites. Don't go clicking willy-nilly on everything that pops up. READ whatever pops up.