Have the same problem on an ECS P4M800-PRO.
Plug in ATX_12V (4pin) with the ATX2.x 24pin and computer boots for 1sec and dies. Disconnect the ATX_12V and the computer powers on, but does not go into POST. Previously, computer was running fine until it needed a windows update--it auto-rebooted and died. Was a CCTV server with about 2 yrs uptime.
Just replaced it with an Biostar board and it POSTs & boots. So, conclusion was it was the motherboard--likely the regulators were shot. What I hate was I had a 3.2Ghz PentiumD (was nice), and thought it was the CPU, so I got a cheap Celeron E3200...somewhat a waste since it appears the CPU is good. Also, the old MB had DDR ram, and the new MB only uses DDR2 ram--so it's off to the shop again
Also, my nice AGP integrated graphics is now a paper weight since the new board as no AGP, but PCIe ... ugh. Oh well, it's going to be another $40+DDR2 memory to fix this problem...
+ Check P/S leds for voltage. Also if you ethernet light comes on, PS is likely ok.
+ If you can POST but no boot, it's memory or CPU (remove then and see if the bios reports the right errors)
+ If you get no POST/Beeps, it's the MB--even if it powers up, it's still the MB.