Really odd power problem

Recommended by Wim's BIOS...

I've googled and searched all over the place, but I can't find anything which looks like my problem, so I am posting here hoping in the collective wisdom.

My home PC has a motherboard Foxconn 661MxPro and runs XP sp2 with 1 gig of memory just fine.

Yesterday the PC failed to switch on, i.e. pressing the button on the front of the case nothing happens. The power supply switch and the power socket switch were ON. So I checked the power supply unit by disconnecting everything, shorting the green and black wires in the socket which powers the mobo, and I successfully ran one of the fans. So the PSU should be OK.
It could be the front switch (plasticky stuff), but here is where something weird happens:
I start with everything OFF.
I switch ON the PSU unit (with the switch at the back of the case).
Then I switch the PC on from the front button.
Nothing happens except a very faint buzz. Then....the moment I switch the PSU power OFF, fans and lights go on for perhaps half a second, then everything dies.

This is so puzzling that I don't know where to start. I haven't put any hardware in, haven't installed any new software, nor have I changed anything in the BIOS for ages. Any idea on where to look, please?

[edwin]

start with the barebone setup and build up your system part by part to see which one is the problem. From the info you give now it sounds like an overload on the power supply, could be from a short somewhere.
http://www.motherboards.org/forums/view ... hp?t=39873
It's also possible your power supply died from bad capacitors:
http://www.badcaps.net

Thank you edwin. Could have been the capacitor thing, but, not having the time to re-build everything from scratch, I applied Occam's razor and I went for a new PSU. Bingo