PC will not power on

i have a compaq 5000t desktop that will not power on. i have replaced the power supply, battery and motherboard to no avail. i know the processor works because i've placed it in another system and it boots without a problem. there are 2 leds on the motherboard (old & new) that light up when the unit is plugged in. so it appears power is getting to the motherboard. when i press the power on button on the front of the pc a 3rd led lights up on the motherboard if i hold the power button in. when i let it go, the 3rd led goes out. the motherboard is for a compaq evo p4 socket 478 processor but i cannot tell the manufacturer so cannot find a manual for it.

can anyone suggest a course of action to repair this pc ?