CPU Fan spins - but no boot !

[kingcomp]

I am working on an old Compaq 5000 series PC (Athlon 1.3 Ghz, 256mb RAM, 40Gb HDD, CDRW & DVD Rom, 250W Compaq PSU)

The PC simply cuts out after a variable amount of time. I had it working long enough to clean approx 500 pieces of spyware, cleaned out extraneous temp folders, defragged etc)

Then it cut out, powered down but the front power light remains on, the CPU fan spins - the PSU fan has stopped spinning though ???

I had a spare, identical PSU which I put in its place. The PC booted perfectly and remained up for about 10 minutes then cut out again.

In both cases, once the cut out had occurred, the CPU fan spins and the PSU fan doesn't.

Won't boot at all now. Power is obviously coming from the PSU else the CPU fan and lights wouldn't be on.

If the mains cord is plugged into the PSU after it has completely powered down, the CPU fan spins and the power LED comes on without touching the power on button.

Any ideas what might cause this behaviour ???

Loathe to go out and buy a brand new PSU only to have it fried by something.

Any pointers appreciated

[ski]

Check each PS as described here:
http://www.ochardware.com/articles/p.../psuvolt2.html

If they're ok, then the MB may be bad. Check it for any bulging, leaking, or ruptured capacitors.