I don't even have anti-virus software installed. They're bloated and annoying bits of kit and in my opinion, just not really worth the hastle if you know what you're doing on a computer. I have my N.A.T router and firewall in place, and I just don't download stuff that I don't trust.
And if worst comes to the worst (it hasn't so far), I've got my backups, my OS cd's and my drivers all handy, so reformatting the computer is no big hastle.
Basically I'd rather stick needles in my eyes than deal with Norton/McAfee/AVG/Generic-Rubbish slowing down my rig and telling me about threats every 5 seconds.