whenever i open up my PC to add/remove peripherals, or do some cleaning...after i put back on the cover and fire up my PC, it wouldn't boot up. I don't even get the memory check screen, the one with the Energy Star logo....just.....NOTHING. but after i leave it alone for a few hours, say, after arriving from school, it works, but sometimes i even have to wait for days before it finally fires up. what could be the cause of this odd behaviour? it only happens when i open up the pc.