kaolinfire

It seems like most people go with whatever's installed on their system, and then they just let it lapse [or maybe they don't--maybe they register it because it's there and pestering them incessantly...] (this is presuming you use windows, at least).
I like AntiVir, personally.

Haven't heard good things about Norton in a long time (since '94?)

Would be nice to use openantivirus, but they themselves say they're a few years away from being something you could trust a system to.