Take out the motherboard, hold it in the air (still connected to PC, just not touching the case), and try turning it on. It MAY work. The reason is that something metal could be stuck behind the board, shorting out the board and causing the self-test routine to cut power to the board when you try to turn it on. This happened when I rebuilt my old Compaq (the motherboard fried so I upgraded in the same case). There were 2 metal screw pins that were sticking out of the back of the case that shorted out around the PCI slots and the board wouldn't POST, and I spent all night trying to figure out what was wrong, but when I lifted the board up (midnight on a school night) it worked....then I had to go to sleep but the next day I took out the pins and it worked fine, booted up, and installed XP just fine.