Try the usual test. Remove the case cover and if the time changes then you need to check your work. The simplest gaffe I encounter is the new builder didn't put heatsink compound on the CPU+Heatsink. They thought it was optional since it worked for a time.
I'm running XP on my my computer that I built myself. My problem is that seance I've been back from a week vacationing, my computer locks up and and will not respond to ANY command after a half hour of no use. AVG found nothing and my start up looks fine. I am considering reinstalling windows but is there another fix. I can troubleshoot most hardware faults but I'm not to good at troubleshooting windows.