yeah what the previous guy said. but before you replace the board - which won't be cheap - try shorting out one of the USB ports. with a screw driver. because one time i managed to do so while it was in use and the power just died and it wouldn't boot up. i was about to strip it for parts when i touched the usb contacts with the outer metal housing of the port and presto it was back to its normal crappy self.
this prolly wont work, but hey aint it worth a shot anyway