I use bitdefender, and its the best ive ever used. Theres no need to disable it while playing games, its so small, lots of options like disabling auto update etc, and its so simple. I also love the way that it takes no ***. Unlike kaspersky which I have used before, if a virus is detected, its deleted by the time the pop up is fully out of the start bar. And if you come across a site that supposedly has virus's it blocks you from going on it, and theres no way you can get onto it apart from uninstalling it. I love it, I think you will too.
I'd recommend getting spybot search and destroy too, best free anti spyware, nice and small, and very simple and fast.
EDIT : I'd also recommend AVG free if you really want a free one, without question its the best free one you can get.