I recently just ordered a new computer w/ vista and did some pretty extensive research on which antivirus to get. assuming you're running vista, which i assume you are, from everything i read ESET NOD32 3.0 Antivirus System is far and away the best. It performs extremely well, catching a very, very high percentage of viruses and, again from everything i read not experience, it by far puts the least load on your system (of the good antivirus software) which is obviously nice for gaming. It also apparently has fantastic customer service. You can get it on Amazon for about 40$ new, which was worth it for me, or shop around im sure somewhere has it cheaper. If you dont want to pay, definietly Free AVG is the best, i use it now and have been happy with it and it consistently tests the best out of all the freeware.

Hope this helps