AVG Free. Haven't had virus issues, updates automatically, and the price is right. Norton and McAfee always seemed to be resource hogs.
I see no reason to waste money on anti-virus when there has never been a virus, trojan, keylogger, or other malware written that would infect the computer. Does not mean it could not happen, but the likelihood of someone writing a virus to attack only one computer is extremely small.
I don't use AV's at all because I never get viruses. I use common sense in terms of what files I download. I've been AV free for 2 years and have never had a virus (I run periodic scans).