Avira is the best FREE av software out there, but it will pepper you with stupid questions when it detects anything, and even experienced users might click the wrong answer to these poorly worded dialog boxes. But it has a good detection rate.
NOD32 is the best "pure" antivirus program for a gaming machine, meaning it is lean and mean, has good detection, good "smarts" to just do the correct action almost all of the time, and it will barely slow your system down.
If you want something that's more comprehensive, Norton Antivirus 2009 or Norton Internet Security 2009 is the best pick, offering good smarts like ESET, an automatic "game mode" that will tell the product to go idle during full-screen gaming, and great detection rates.
I know this because I tested all these products for the July issue of CPU Magazine.
-Llama