Computer turns off when CPU power connected

Hello, A few days ago my custom-built system stopped working. I've tried various things but have come to a point where I don't know what else to try. Any hints very welcome.
When I try turning the system on the fans start rotating and then everything turns off again (no RAM counted, no beeps).
Here is what I did so far:
- Checked the motherboard for obvious faults: nothing found.
- Taken out/moved around/swapped RAM modules: no difference
- Replaced the CPU and CPU cooler with identical parts: no difference
- Tried a different power supply: no difference
- Unplugged the CPU power connector from the motherboard: Now the fans keep running. Plugged it back in: original problem.

Does anyone have any ideas?

My system:
motherboard: Asus M2NPV-VM
psu: Seasonic S12-380HB
cpu: AMD Athlon64 X2 EE 3800+
ram: 2 x 2GB DDR2 RAM
onboard graphics

Thanks loads!

 

Any bad weather there ? Like lightning ? Sounds Like a power surge made toast out of your motherboard..

 

Not that I remember, and no other device with hiccups either. How can I ascertain it's the motherboard? Would you suggest the CPU power connector is damaged in some way?

 

No, It is more than likely a blown transistor...

 

Thanks for that assessment. I didn't see any suspicious transistors but I'll take another look. Maybe I'll get a new board, then...

 

Replaced the motherboard, everything back to normal. Thanks Ghostman 1.