I have a Dell Dimension e510. For the last few weeks, it's been randomly restarting, usually if it's been idle for a while. This morning I turned it on, and it restarted itself after a few minutes. It did this a few times, until the length of time it was actually up and running dwindled down to me pushing the button to turn it on, it lighting up for a few seconds, and then dying again.
Everything else seems to work just fine, I've hooked everything up to my old hard drive.