I couldn't write a more accurate title.

I am just unable to start the computer by pressing the Power On button.

I checked the Power Supply and it seems to give good voltage values in every pin. And this is not a BIOS malfunction because of bad overclocking or anything that may come to your mind.

And I will tell you why.

It happens that EPS (or any ATX-based) power supply has the ability to be powered-on by the Motherboard by jumping the 13th pin of the 24-pin-ATX-connector to COM/GND.

I did it, after pushing the power on button (without any visual response) and, pwhaa! The machine turned on.

I was able to read (and even write, if I wanted) BIOS values and then start any OS installed.

Machine starts, so it's not any kind of misconfiguration.

It seems some hardware related.

I am able to power the machine on only if I already pushed the power on button. Though pushing it without jumping the 13th pin to ground for a second does not power the machine.

Of course, jumping the pin without pushing the power on button does not tell the motherboard anything, so the computer would not start up either.

It's as if the logic that connects the power button with the 13th pin derivation to GND was unable to be activated.

What can be the issue? How can I solve it?

My configuration is as follows:

CPU: AMD Phenom 9850 X4 Black Edition
MB:  ASUS Formula II AM2
RAM: 2x2GB Corsair Dominator 5-5-5-15 2T @ 1066MHz DDR2
     Tested also with only 1 module
GPU: 2x XFX nVIDIA GeForce 9600 GT XxX Alpha Dog Edition @ Core: 540Mhz [SLi]
Power Supply: Xilence 700W (ATX 12V 2.3 / EPS 12V 2.92 compatible)

PS: I know the machine is like 2 years old. I hardly use it now, but my parents do.