Like others stated, if it only does this with AVA, it's probably a game bug. The fact that you can CTRL-ALT-DEL and get a response suggests it's not an actual system problem. Otherwise it would just freeze up solid or BSOD.
You might check the FAQ or Support Forums for AVA and see if others with nVidia cards have reported this problem. There may be a fix for it, or at least it may be a known problem.
EDIT: By the way... a Google search found an AVA community of gaming. But not an "AVA" game.http://ava.ijji.com/index.nhn
Is this what you're playing with? Alliance of Valiant Arms?