I haven't had a real virus infect my system since probably 98 or 99, I think it was some virus that got in when browsing network files, man I'm not even sure if I ever got infected then, because I specifically remember Norton making my system speaker screech and dropped it to a red screen, similar to the BSOD of 9x, saying that norton had caught a virus trying to infect me. So maybe never?
I'm going to put Media Center on my main comp this weekend, I dont' think I'm going to put any AV on it. Why? Because I use Opera, don't visit shady sites, and don't use email outside of gmail, and I don't click on links I'm not expecting that get sent from "friends" on IM.