Stability Issues

I have an advent 1 gig pc with Tnt2 m64 graphics, but the computer is really unstable when playing games. The only game its fine with is championship mamger which uses no graphics, which tells me it might be a graphic pronlem. I have the new detonator drivers from Nvidia and direct X 7a but still no joy. I have been told to update open GL but dont know where to start.