Fixed a computer for a friend with a strange problem. You would push the power button an it would come on then after 3 seconds turn off. First thing that would come to mind is that some motherboards turn the power off to the computer if the processor fan does not spin on startup. The fan was spinning within those 3 secs so that was not it. Then I thought I could be a possible power supply problem. After putting a new power supply in the computer continued to do the same thing. So finally I listened closely and could hear the power switch actually turning the computer off or getting sent a signal to turn off. So getting another power switch and hooking it up did the trick. So after initally looking like power supply or motherboard problem turned out to be a power switch problem. Weird!