What type of CPU is it? If that's an Athlon Tbird, that's warm, but shouldn't cause lockups. If it's a Pentium 3, that's probably too hot.
I wish I had some information about your particular system, but more often than not a CTD from a game followed by a system lockup usually indicates a few things. First (most often) the video card is overheating. Next would be insufficient power supply for the system. Last would be a driver issue, but since you just reloaded windows this becomes less likely.
My guess is the problems are heat related. The easiest way to test is to take the sides of the case off and get a fan to blow cool air into the system. If the lockups cease you know heat is part of the problem. If they don't stop, your powersupply could still be to blame. Most monitoring software can check the voltage levels coming off the power supply. Of particular interest in your case would be the 5V+ and 3.3V levels. If there are spikes or the levels are not within a few tenths of a volt I'd be concerned.
