I think the most popular antivirus is Microsoft Security Essentials as it is free, very lightweight, stays out of the way until needed, and catches most things but has a hard time removing things if you actually catch something. I personally used it and it works like a charm.
LMAO...A couple of days ago I got a call from a guy that could barely speak english. You've heard about these calls; says he is calling from Microsoft and there is something wrong with my windows. He wants you to take all these steps on your keyboard and the next thing you know your computer shuts down and there is no fix for it. Well, I said, "This is a scam, isn't it?" He asks, "Who told you that?" Just like he would know the guy. Like we travel in the same circle of friends. Pretty funny.