i've never had antivirus software running on my system. the most extreme i've ever gotten was running firewall software to micromanage ports, which i now do with my router anyway.
my protection system is quite comprehensive: don't do anything stupid. keep an eye on the running process list and it's tally once or twice a week (from a fresh install of windows to my regular software, the tally is always 30) use opera since 2002, the most unpopular browser on the planet. check windows update once a month.
wow! amazing! i do periodically scan (we're talking like once a year) my system and have never found anything remiss. i also format once a year or two just out of habit. also, fuck norton.