Emachines T2824 Dead System

i've been trying to fix this computer and i cant think of anything else that is wrong with it but the motherboard. i've tested the PSU, i've checked the whole computer to make sure that there wasn't any short circuits in it, i've checked the RAM to make sure that was ok and i even checked the cards like Video, Ethernet, Modem, etc with no luck. when i plug the computer in, the light on the motherboard turns on but when i unplug it, it immediately goes off. i know that motherboards have capacitors that hold the voltage at least for a couple of seconds, could that mean that their is something wrong with the motherboard?