The little things...(how I brought back to life a dead Computer)

So....my sister's computer died. It crashed with a bluescreen, she rebooted, it crashed again at the motherboard logo screen, and after another restart...it wouldn't go past a complete black screen at system start up. It would power on, and turn all the fans, and drives, but nothing else
No beeps, no motherboard logo/POST no nothing. Even if I reset the BIOS, removed some/all RAM modules, disconnected all drives, removed the graphic card....nothing....


I thought it could be the PSU, or the MotherBoard/CPU.
I tried booting my old Celeron 400Mhz with the Pentium4's PSU and it worked there. Plus, one thing puzzled me about the Motheboard being dead. I have on that PC a 7800GS VGA card, and it requires external power to satisfy it's demands (it needs a 12v connector apart from the power it drains from the AGP slot).

If I turned it on without the external power...it would let out a long continuous beep. Maybe the motherboard wasn't really so dead after all.
That confused me even more, odds were it wasn't the PSU since it was able to boot a lesser system, so I was almost sure about it being MB/CPU related.
Suddenly on one of my tries...it booted...but since I had the keyboard turned off...couldn't do a thing..and since I had reset the BIOS the right disk wasn't configured to boot.
I quickly turned the keyboard on again....booted...and...black screen again.
Then on a stroke of genious....I figured it out! If I disconnected the keyboard...it would boot...otherwise, it would present me with a nice black screen at system power up. I got my media center's USB keyboard..configured the drives..and...it's in working order again.
In the end...my analysis was accurate, it was the motherboard as I suspected. Probably the PS2 port died...and whenever there's a keyboard attached to it...it halts the system dry.

Bought a USB keyboard today (my sister got a blue backlit version for just 9€), and now all is well...

But even if it was the motherboard's fault...truth is....my computer completely died and was almost trashed.....because of the KEYBOARD! One silly 10 euros piece...was killing it dead at start...and leaving me completely clueless...!
(Next time I know....include the keyboard in the error debug procedures when there's computer problems).