PC Will Not Boot!!

I have never seen this problem before. My Customer has a Systemax PC AMD K6 533 CPU. 64
Megs of Ram. Here’s the problem, when you turn the PC on it stays on for a few seconds then shuts off. I have replaced the sorry 145 watt Power supply with no success. I have also ruled out the Memory. The Motherboard does not appear to be shorting out. Any help would be appreciated.