Here's the deal: a few weeks ago, my computer stopped booting up. I opened up the case and noticed the led on my mobo that's usually always turned on was off, and guessed the problem was probably my PSU (a brandless 420W module). I then went to my local hardware store to have my PSU checked out. Indeed, it turned out to be the problem so I bought a new one (Nano 500W from TopPower) and went home.
Once the component was installed, I promptly booted up the computer and that's when I thought seeing, out of the corner of my eye, a blue flash coming from my motherboard. Since, my computer turns on but doesn't send a signal to any output (mouse, keyboard, screen, etc.) and when I try force closing it by holding the power button, it turns right back on of its own accord 5 seconds after.
Blue flashes are never good and will let the magic smoke out! Yes it does sound like a short circuit but I don't think that the PSU caused it. Some component on the motherboard probably a cap failed and created a short circuit.
I don't understand what you mean by saying "... and when I try force closing it by holding the power button, it turns right back on of its own accord 5 seconds after. "
When you hold down the power button for about 5 seconds, the power is immediately cut from your computer, no? I know it's not recommended, but it's the only way I was able to turn off my computer. Even then, every time after cutting the power, the computer turns back on on its own without me pressing the power button.
Yes, holding down the Power button for about 5 seconds is known as a hard shut-off. After doing that the computer should stay shut off.
If the computer turns ON by itself it means that there is a short in the 'Start' switch. The two leads from the start switch go to two header pins on the motherboard (except in some computers like Dell where it goes into a multi connector).
To isolate this problem, carefully unplug the two tiny connectors that go to the motherboard start (Power) pins, and then use a flat blade screwdriver to briefly short these two headers simulating the closing of the start switch. If by doing so your computer works fine, then replace the start switch with a new one.
I tried shorting the switch pin and the switch is NOT the problem. It's the same as before: when I start the computer, no output turns on (screen, keyboard, mouse) and if I do a hard shut-off afterwards, my computer turns back on 5 seconds later.
Note: my computer doesn't start this sequence until after I power it on for a first time (another proof it's not the switch).
EDIT: I found the problem! The PSU 4-pin into the mobo wasn't plugged in well.