When I got home last night the computer was off but the green light was still on. I had to cycle the power, not just reboot. When it happened again after only a few minutes, I figured that it had overheated. (1) Modern mobos have thermometers and shut off when the CPU gets too hot. (2) I had my AC off and windows open in the apartment for a few days to save energy. (After a while, you get used to the heat. And the natural layer of sweat.) (3) The computer seemed quieter than normal (this is hard to judge); maybe the fan was out.
So I went to get a new case fan. This one is much quieter than the old one that came with the case four years ago. While I was at the store, I picked up some comressed air, figuring that dust was the likely culpret. I'll try and clean the case every time I open it from now on. Maybe I should think of compressed air a s acleaning product, like soap or windex: something I should always have on hand. Maybe I should make a point to clean every computer every six months.
I hope this computer doesn't die in the next few minutes. Then I would have to find the real problem.
UPDATE 2005-08-15: Turn out that the power supply needed to be replaced. I'm not so competent.