I don't have an AV installed (except perhaps Giant Antispyware, which is pretty good at nabbing errant behaviour in IE). Every few months I might run housecall, but it never turns up anything. But then, I don't download ripped apps (I'm not a BitTorrenter, nor a P2Per) and if I don't have complete faith in the author of a product, or the product author's liability insurance status, I won't let it near my PC. I don't have a firewall either, except NAT on the router, so my PC's performance is pretty minty when all is said.