Computer turned off by itself

Could be a few things.

It could be the motherboard is messed up a bit. One time I had a power surge, the computer wouldn't turn on, thensuddenly it would but the computer would automatically restart even after I had selected power off (and things got worse from there).

It could be the fan inside got damaged and so when the computer begins to overheat, it shuts down (I thought though that it would just shut off).

Could also be a program inside got damaged and so it THINKS Windows is suppose to be shut down.

And then, as always, it could be a fluke.

Best thing to do is to grab the Ultimate Boot CD and run it. Do some diagnosis on the ram, motherboard, hard disk drives, etc and see if there are any problems. If not, watch it for a while just in case.

Jam it back in, in the dark.

Let's try a few things.

First, as for the fan, if it turns off by itself again, try to get into the BIOS and find out the temp of the computer at that time. Record it. Have the computer shut off for ten to fifteen minutes and start it up again, immediately getting the temp again. If it's a significant difference, then it may very well be your fan.

However, my feeling is, especially with prior experience and it blantantly turning back on, is that the motherboard is fried. I would begin to look for a new one to replace the old one inside. Get your old HDD out and try to (as people mentioned) put it in an external closure with another computer to see if you hear the sound/get your data off of it. If you get a new motherboard, most likely, you'll need to reinstall Windows.

There's nowhere I can't reach.