One antivirus that's extremely unobtrusive and doesn't hog your system resources is called Eset NOD32 Antivirus System. It's very efficient, extremely quick and actually quite cheap! If you buy it online from their own store, that is.
They have a 30 day demo for you to try out, to see if you like it. I'm sure you will, as it is perfect for gamers. Not only is it MUCH smaller than most of the AV's out there, it's also very easy to turn off if need be. You probably won't have to though, since it's so small and discrete. Highly recommended!
Edit: Found a comparison against the top names in the AV business:
http://www.eset.com/products/compare.php
Check out the speed and footprint graphs!