Game Crashing - Not just freedom islands

[Quixs]

I bought Kings bounty:TL and AP through Steam about two weeks ago. Everything was working great until a few days ago. Now no matter what I do, the game will only last about 5 minutes running fine before it freezes and either resets my computer or hard stalls forcing me to cut power. This only happens when playing these two games. Any help would be great.

Running Windows XP
3.2 GHz processor
Nvidia Gforce 6800
3 Gigs Ram