James wrote:

I connect the system to the mains. Turn mains on, switch power supply
main switch to "on".

The power supply and CPU fan start spinning though the computer has
not yet been booted up (this indicates the power supply is powering
the motherboard as soon as the PSU in switch on as the CPU fan is only
connect to the motherboard).

Press the power button on the case, XP boots fine.

Select shutdown from XP, the system shuts down, the disk stops
spinning, all display has disappeared, both CPU and power supply fan
still turning at full speed. Switch of either power supply or mains to
stop fans.

Any idea ?

What voltage is on Pin 14 (PS_ON) of your PSU/MB connector? If it is
2.0v or greater, PSU and all fans should be off. If this voltage is
2.0v or greater, and the PSU is *not* off, then the PSU is defective.

If Pin 14 (PS_ON) is 0.8v or less, the PSU and fans should be ON. If
Pin 14 goes to less than 0.8v as soon as you power the mains, check
the +5vsb. If it is low, PSU is defective. If it is at +5v and Pin 14
is at less than 0.8v, then there is a problem with the front panel
switch or MB.

Good luck.