PC won't start

[Dr. Boogie]

Last night I left my PC on to finish a download, and this morning I found it off. When I tried to start it back up, it got as far as the Windows loading screen, and then turned itself off. For some reason, though, the keyboard remained on, albeit locked up.

So I opened it up and cleaned some of the dust out. That seemed to work, and it got into XP, but a few minutes later, it turned itself off again. I tried cleaning it a second time, and now it doesn't start at all; you push the power button, and the power LED goes on for about a fraction of a second, but nothing else starts.

Has the power supply gone bad, or is it somehting else?