basic power problem

How do you get a motherboard and an atx power supply to come to life?

I hope this is not too obvious a question. But basically I've never built a
pc before so I don't have any experience. I've fitted extra memory, drives
and cards but I've never built one from scratch.
Anyway here is the background info. Recently I acquired a pc that had died.
It wouldn't power up. There were no beeps or fan activity from the power
supply unit.
The computer uses a standard atx power supply and motherboard. It was taken
to a pc repair shop and they tried it with a new power supply, which didn't
work, so they concluded there was a deeper problem.
Anyway I bought a new power supply and tested it with the new one, but
nothing happened. I had the same problem. However the fan on the power
supply came on very briefly (1 sec) then all activity would stop.
To make sure the switch on the pc wasn't faulty I shorted the power jumper
and nothing happened. So I thought the motherboard must be faulty.

I bought a cheap similar motherboard off eBay and I cant get this one to
work wither.

My question is If you have got say just a motherboard and an atx power
supply and nothing else, no hard drive, memory cpu etc. When you connect the
power supply to the atx power socket on the motherboard should the power
supply come to life with the fan spinning if the power connector jumper is
connected to a switch on the front of a pc case?

The motherboard I bought is a 'Gigabyte' GA-6WMM series Intel 810 AGPset. If
that means anything. I placed it on a table (with nothing connected to it)
and connected the power supply to it. Nothing happened (as would be
expected) however I looked through the manual and found the PW(Soft Power
Connector) jumper. Am I right in thinking this should connect to the pc
switch?
I shorted this with a screwdriver to simulate a switch action. But nothing
happened. (I was very disappointed).
I then attached various things (cpu memory, pci cards) and placed the
motherboard in the old computer case and connected the switch connector to
the PW jumper. But again nothing.
Have I missed something. If you were building a pc from scratch and had just
bought a motherboard (as i had) how would you tell if the motherboard was ok
before assembling the entire thing?
Thanks