To expand on Bob's comment, modern ATX style PC's (basically all of 
them) have a momentary contact power switch. It must make contact only 
while being pressed. This causes the PC to turn on. As an emergency 
shutoff measure, if you hold the power switch in for 5 seconds, it will 
power off.

The Original Poster (OP) mentions that it shuts off 5 seconds after 
turning it on, so this suspiciously sounds as though the momentary power 
switch may be sticking closed. This has actually happened to me once 
before, and caused these symptoms.

Russ Poffenberger
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bobwarasila wrote:
> Are you sure the on/off switch is latching?
>
> Bob Warasila

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