You may want to combine some elements of the suggestions here, with some decent but more time consuming habits.
First, you could easily install a second OS on the system and dual boot. Yes, that seems like a pain in the buttocks (at first) but it means you can do all your web surfing from a version of Linux installed on the system (without having to worry about losing resources to an AV running in the background). Unfortunately, using any tabbed browser is just like using multiple instances of a non-tabbed browser... the more tabs you open, the more resources the browser is going to consume. However, this now enables you to use Firefox or Chrome on your system, in Linux.
Then, when you want to play games, you just reboot to Windows. Now, if you aren't going to expose that computer to the internet through surfing or email (etc.) then you can most likely do away with an AV suite. I know... the horror. Except... if you are NOT stealing games off the net, and you are not browsing the web... where would the infections you are worried about be coming from? This way, you can have your Windows, and get away without a resource hungry AV.
If you insist, you can easily have Clamwin AV on a thumbdrive for when you want to scan and clean the system. It would only be running when you inserted the drive into the machine, and ran it manually... but again... if you aren't exposing the machine to infections, what is there to clean?