I haven't used an antivirus program in nearly ten years, and the only time I've ever had a virus was on a school computer, it was the blaster or sasser worm, I forget, and I *was* running an antivirus program at the time.. I forget which one it would have been.. I used F-Prot back then, but I think at school I was running whatever was preinstalled on the computer there.
Lately I've been getting a bit paranoid, since I started working at a computer store and started seeing dozens of people per week just crawling with scads of viruses, so I've been installing an antivirus and doing a scan every 6mo or so. Last round found no viruses except what it considered "hacking tools" (omg, vnc is a program for remote desktop, therefore it MUST be evil.. omg, you have a password recovery tool for that MSN password you forgot last year, this is a huge security risk!, omg you accepted cookies from LPF, therefore you're being tracked and your security is totally compromised!).. Anyways, the best antivirus I've used lately is probably NOD32.. It's lightweight, thorough, and gave me very few false positives. My least favorite is probably Norton Internet Security.. Pretty much half of the computers that come into my store riddled with viruses are running an up to date version of Norton, and when I uninstall it their computer doubles in speed.