BleepingComputer.com: Computer keeps restarting

As my subject title says, I'm having a problem with my computer restarting itself while I'm working on it. I don't think I could say it's "random" as it seems to be when I'm opening up a window within a program or something like that. For example, I want to download something and as soon as I click on the Download button the screen goes black and then the boot info does it's thing.

Here's what I've already tried:
- I put in a brand new hard drive and reinstalled windows (assuming it might be some kind of virus or software issue). Still had the problem
- So I put in a new power supply unit with higher watt rating, figuring it might be a power issue (as it seemed to happen when I was opening something, so I thought maybe the unit is going bad and can't handle the added drain). Still did the same problem. Also, I don't do much on the computer that draws power anyway. Only internet, accounting program and MS Office. No wild gaming, videos, music, etc. Just boring work crap.

I don't think it's a heating issue as it will do it within minutes of the computer being powered up (and besides, I've had the enclosure off the computer while running it, as well, and still did the restart at some point).

The computer is a generic (no name brand) pentium 4, about 3 years old running windows xp. It's run fine until recent months.

So after that, I have no idea what could be causing this problem. Any suggestions would be greatly appreciated.

Thanks!

If it's shutting down without warning or no error message, I would still suspect heat
Even with the side off, it the heatsink is packed with lint and crud, it will overheat

Garmanma, I think you may have hit the nail on the head. I took off the CPU fan and heatsink and there was indeed this black caked-in crud on the surface of the heatsink and CPU. I cleaned both off nice 'n' shiny and put back together and didn't have a single restart issue the rest of the day. And I did all the sorts of opening up things that would cause it to restart before. I'll see if that continues, but so far looks like you are right on. Thank you so much!!!!

That black caked on crud you mentioned you cleaned off of the CPU & heatsink was old thermal compound.
If you clened the CPU & heatsink until they were nice & shiny you need to get some3 thermal compound, remove the heatsink, put a thin coat on the CPU & then put the heatsink back on.