I have never had a virus found. But I still install AV as the first thing, I like the sense of security, (i am using eset smart security these days - 50% student discount here in Aus anyway)
I only run a full scan very occasionally and every so often, every month or two, will run spybot or Asquared free just to make me feel good

And an antirootkit (blacklight and/or panda) They never find anything either apart from a few cookies, and i clean my cookies regularly and only keep the ones I want/need.