Stryker
Knight
I use Avast myself (Win7 x64, 2.66 C2D processor, GTS 250 graphics card) and haven't had the slightest bit of stuttering other than that induced by other programs running in the background. There's a couple of reasons I ask: for one, spyware/viruses can severely affect the game's performance, and for another some antivirus programs can be nearly as bad. I've seen antivirus programs that scanned every single hard drive access request before allowing it, severely impacting the performance of programs that created a lot of hard drive activity.