Computer Randomly Rebooting

I mentioned this in my FTP thread, but I've been having strange issues with my computer rebooting on its own randomly.

However, this is usually during some intensive work, such as deleting a large number of files or attempting to run a game. For example. I can't play Portal. It won't let me load the game without rebooting.

Sometimes, it'll reboot when nothing's happening, though- again, it's extremely sporatic.

However, the big thing I've noticed is just before it reboots, it makes this high pitched noise. I can't really describe it other than something a high-pitched whine. It's not very loud, and it's most definitely not mechanical. This is, pretty much, almost identical to the sound of a TV left on with the mute or no signal- that sound it makes, if you can hear it.

Does anyone have any ideas of where to start with this?

How ya doing, buddy?

I don't mean to unearth this topic again, but I wanted to thank you guys for the help. It was the PSU, and I've gotten that taken care of. Just have a few last minute fixes, and it should be up and running again.

Thanks, guys!

There's nowhere I can't reach.