PC won't turn on

Hey, I've got a problem which goes like this:
A couple of days ago my PC just shut down without warning. The power button, the network interface and the Mobo power LEDs kept on glowing, but that was just it. Since it didn't react in any way to me pressing both the power and the reset buttons, I turned off the power switch on the PSU. After turning it back on after some time, the PC wouldn't turn on at all. The Mobo power LED would glow, and when I pressed the power button it would glow faintly and briefly, the MMC reader LED would light up fully and then dim and the network interface LEDs would blink. The processor fan would start working but only for a second or so. I tried replacing the PSU but it didn't change a thing. I also disassembled the whole PC, leaving only the Mobo, the PCU and the PSU in, and it still wouldn't show any signs of normal life.
A friend suggested that I press the power button several times, since it (supposedly) works in some cases. After doing that, I noticed a new LED light up, except that it wasn't a LED, but a chip of some sort .
Well, at least then I could definitely say that my Mobo was fried. So today I went out and bought another one, a Gigabyte GA-EP35-DS3L. After assembling the PC back together and turning on the PSU switch, the power button and the MMC reader LEDs immediately start glowing, yet no fans or any other parts seem to be working. I'm hesitant to start pressing the power button repeatedly, since you now know what happened the last time I tried that
Anyway, does anyone have any ideas about what the hell is wrong with my PC?
Thanks in advance

Did you try bench booting either board (outside of the case) to rule out a short in the case or switch wiring ?

On the new board make sure the additional power lead (CPU) is connected to the main board (this may be a 4, 6, or 8 pin connector)

I've only tried starting the PC with the (first) Mobo, the PCU and the PSU, to no avail. I also thought that there might be a problem with the case but why (and how) would it occur out of nowhere, let alone fry the board?
I've also double checked the new board with another PSU, nothing happens

I would remove ALL the drives (hard drives and optical drives) and anything else (any other add-on cards) from the system..including analog modems, NICs, sound cards etc. et al

Nothing but the memory modules, keyboard and mouse...and see if it will post (outside of the case)

it is possible that one of the circuitry on the drives has been corrupted or experienced an electrical failure that could be causing a short...

Will do. But how do I start it outside of the case? Or do I just take the board out and leave the power/reset/etc wires from the case connected?

if you remove all the case panel leads...you will need to locate the power switch pins on the mobo and if the PSU etc. is connected all you need to do is short the (two) pins for a second and it should start...

use a paper clip or other conductor and touch both pins at the same time..