Unexpected shutdown

My HP Pavillion a1140n running XP Media Center Version 2002 SP2 is unexpectedly shutting down. This started happening a few days ago. when I would leave the computer unattended for awhile, when I returned the screen is black and hitting space bar or mouse click does not bring it back. The "power" light is lit on the computer but it is unresponsive (no fan noise etc). I can get it restarted with a cold re boot and it is fine. Several weeks ago it was making a loud noise, like a bad fan or maybe the hard drive "grinding" or "whistling". I tried resetting the power saving setting. Currently its set at "never" for turning off CPU or monitor and never for standby or hibernate. Today, it went black while I was using it – IE7, MS Outlook and Critix were open. There was no indication of any power problem - no flicker of house lights etc; but it went dead as if the power were cut (except the "on" light of the computer is still lit). Any ideas as to what is causing this. It's becoming more frequent. Thanks.

Whats the temperature inside, you may need to look at heat monitoring of your system

Thanks; I think that is the problem I took off the side cover of the computer to give it more air; it is inside a cabinet and gets pretty hot; I haven't had any problems since.