I haven't written much games that involves real-time animating, but isn't the updating usually tied to a timer?
If so, I would think that there should be some amount of idle time, since generally there's no need to update more than 60 times per second (movies are 24fps, and 30 fps should work fine for most games). The game would've either slowed down or dropped frames if it can't handle updating that fast, and since I haven't seen neither, it would seem that the updating takes less than the interval between frames.
Maybe it's just the strain of having to update the screen? Anyway, it also sounded like the machine in question didn't have adequate heat dissipation, so maybe it's better thought of as a hardware issue rather than trying to get around it in software.
Peter, if you think there are places in your code where you're doing polling loops (besides the standard windows message handling), explain to me what they are for and maybe I can tell you how to avoid the polling.