dell dimension c521 wont stay on. comes on, says did not shut down properly and I choose normal start up. it starts up just fine and windows comes up and my desktop and all works but computer only stays on for 4 or 5 minutes then shuts down completely. I have replaced the power supply and it made no difference. Can not hear fan running at all. Wont stay on long enough to do a system check, but I did get into settings and changed the options to never shut off or sleep. Still made no difference. Any ideas?