PC Automatically Shuts down on Boot.

Hi!

I've got a PC that is about 3 years old, it has good specs and usable for all games.

The PC has worked perfectly these years all up to this morning, The PC starts when I press the power button, but after (this varies) 1-30 seconds the PC shuts down, just like someone pulls out the power cord.

What is most likely the problem? what can I test to further my investigation?

I've tried restarting the BIOS ( Removing the battery)
Clearning the entire PC
Checking All cables
Removing ram, inserting them again
Removed all USB devices while in boot.

So the "obvious" or "most likely" problem is that there is something wrong with my Power supply, but after some google searching ( which haven't solved my problem ) says it's fast to blame the PSU but not often the problem.

Any tips would be much appreciated

(Desktop Win 7 PC, 16 GB ram, Decent graphic card, prosessor etc - Don't have those specs infront of me)