I have a HP Desktop that fails to boot up. I have a power led on the back that glows when first plugged in, but goes out when I push the on/off switch. If I unplug it for a few seconds, the led flashes once. Then when I plug it back in the led will stay lit until I push the on/off switch again whereupon the light immediately goes out and the computer fails to boot up. I'm sure it must be a bad power supply (short?) or the switch. Is there a way to test which it is myself or do I need to take it to a shop? Any help would be appreciated.