maybe the warning is disabled in the BIOS
There's rarely a warning unless the overheating problem isn't very severe. 99% of the time the computer will just turn off. I suspect either your CPU or your motherboard, but since it's only 2-3 years old unless it's a Gateway or an eMachines, I tend to think it's a heat issue. Either the CPU fan has died or the thermal paste has thinned. Or maybe the processor is just shot. It could always be the PSU as well, but I doubt it.
And Machine Slave...I suggest you refrain from just repeating what you hear online unless you're able to verify it with your own experiences. Only give advice if you really know what you're talking about. Computers aren't like they used to be. They're very diverse and complex. Just because your machine does something doesn't mean it's the norm or that another computer will act remotely like it in the same situation. My experience comes not only from the ~200 computers that I personally own, but also from seeing 6-12 client computers every day and dealing with 6-12 different people's problems.
As for Everest, the program is more complicated then it needs to be for your particular issue. Go download this:
http://www.afterdawn.com/software/system_t...n/core_temp.cfmAlso I was going to mention this earlier, but it's possible that the computer is blue screening but just restarting. The computer is set by default to restart if it encounters a blue screen. The blue screen will likely flash for a fraction of a second before the computer restarts. But since you're saying that it's actually turning OFF and not restarting, I tend to believe that there is no blue screen and that your problem is definitely related to heat. If you'd like to check, follow these steps (for windows Vista):
1. Right click on "computer" (found under the start menu) and go to "properties"
2. Over on the left side of the screen there will be an option that says "advanced system settings." Click it.
3. Click on the "settings" button under "startup and recovery"
4. Under "system failure" uncheck "automatically restart"
5. Click "Ok" and "Ok" on the two open windows and close the "system" window.
If you follow these steps and you get a blue screen, let me know. If you do then this may be another issue. But as I said, since it's shutting down and not restarting, I'm pretty sure it's a heat issue.