Computer on, fans go, then power shuts off - help required

Hi,

I have a problem that in all my years of tinkering with pc's this is the first time I have seen this.

The problem is that when I try to turn my pc on the fans start (case, cpu and graphics card) but then they shutdown 1.5secs later. I try it again and the same thing happens:

Spec:
PSU: 600W
Mobo: MSI K9N Neo version1
CPU: AMD Athlon 64 6000+
RAM: Corsair Value 4gigs (4x1gig sticks)
Graphics: Radeon HD5770
OS: Vista 32bit
Other stuff: 2 case fans, CD/DVD drive

The system was last upgraded 12 months ago (cpu, graphics card and 2 new sticks of ram) and was working fine, the rest of the components are about 4 years old.

As an engineer the fact that it comes on and then off suggests to me something is shutting off either for protection or not receiving an 'OK' signal or similar.

After browsing for solutions I have tried the following:
1) Tested the PSU for voltages, all ok. I have even just unplugged the 20pin connection from the mobo and put that into the PSU tester. The tester completes the circuits and all the fans then come on and stay on.
2) Removed all connections and peripherals including ram from the mobo minus the 20 pin plug to at least get it working but still the same problem.
3) Then raised the mobo up on crued cardboard legs away from the case.
4) Have reset the CMOS (at least attempted to) by removing the battery for 20mins.
5) Tested the battery for voltage. Measured at 2.97V
6) Have checked the CPU heatsink fitting by removing and resetting. Have not removed the CPU though.

When it first started to go wrong I could eventually get it going by hammering the case switch several times in close succession. However this was hit and miss as sometimes no matter how many times i tried it just wouldnt start. During this time I checked the votages to see if any dipped while turning on and the 12V, 5V and 3.3V showed no sign of load issues.

So abit stumped and before I go and get a replacement mobo (if I can find one) or psu I would like to check ive exhausted all avenues. Ta.

PS have put it under motherboard only as a guess.