Computer do not want to boot. Not even to BIOS.

Hey guys.
I'm speaking of my rig here. Yes, it doesn't want to boot anymore. Let me explain the situation.

Since a while, maybe last month, my computer started to close and reboot on its own. Then, I was surprised so I checked to see if my fans were running, the temperature of the CPU, GPU, then my HDD's smarts value, etc. Everything was fine! But then, it stopped for a while until yesterday evening where it stopped 3 times in a small lapse of time. The third time, I had to do an electric reset then it booted again and everything was fine until I closed my PC and went to sleep.

So, I booted my PC again today and all was good for a couple of hours. Then, it stopped and rebooted 2 times and the 3rd time was the final nail to the coffin.

When I press the power button, there's no signal to the monitor and the hard drive activity LED is staying on. That's about it. No BIOS is shown, no bips either.

I disconnected it. Tested all my hard drives inclusing the SSD with a USB adaptor and they all showed up on my mom's PC just fine. I thought it was my graphic card so I took my old GeForce 8500 GT and swap it with mu GTX 460. Same result. What I can say is that all the fans power up, the LEDs too and the small LEDs beside the RAM slots too.

I'm stumped. It's either the motherboard or the CPU at that point.

What do you think?
Help!

Thanks in advance!

P.S Using my mom's PC.
P.S2 I don't have any another PSU, motherboards, CPU and RAM to test with...

EDIT

There were no BSOD or any other errors when restarting was happening. Just pointing it out.

I do but it's only 500 Watts my current is 700 Watts.

I was thinking the same don't count out the Power supply and 500 should be enough to post without HDD's DVD's Etc. plugged in.

I will try with my friend. I hope he can come over today. My self-confidence is actually very low. (Actually stressed atm)
Will give update once we test that out.

That 500 w power supply will be perfectly fine to test with, especially if you do it the "best" way (see below).
But it won't hurt to try and boot with the smaller power supply before you try the "big test" below. Windows might complain, but the boot process will go through if that is the problem.

The best way to diagnose a non-booting PC:

• Disconnect everything (all cables, drives, etc) from the rear of the PC, and the motherboard.
• Remove all cards, including the video card.
• Remove all the RAM.

You are now left with just 3 components: power supply (500w) connected, motherboard, and CPU. For an even more thorough test a lot of folks remove the motherboard and place it outside of the case.

Then plug in or turn on the power supply and start the computer.
You won't see anything, of course, but you should get some error beeps. This is good. They are telling you that the boot process is proceeding normally and "no RAM is found".

If the MB is out of the case, now is the time to shut down and put it back in. Watch those stand-offs! Then boot again to retest for the "no RAM" error.

Power down and then install one stick of RAM in slot A1. Boot again. More (different pattern) beeps mean "no video card detected". Good. This means everything is working as it should.
If you don't get the BIOS error beeps at any point then you have found the problem hardware.

Power down and then install the video card, and connect the monitor. Boot again. There should be no error beeps at this point and you should see the BIOS screens on the monitor. It will stop when it gets to "no boot device found".

Now just keep adding components back to the system, one at a time, to test to see if the symptoms return. Start with the remainder of the RAM - one stick at a time.

Hope that helps.

Update.
My Antec 500 Watt is not enough. My motherboard needs an extra 8-pin and the 500 Watt Power Supply doesn't have one. Only 4s. So, I disconnected the whole power suplly and I will test it on my friend's PC.

Nah, jumanji... I don't think that it will help... the most it can do is that the BIOS settings will be lost and the Windows time won't be the real time. (will contantly lose time, if turn off)

@TVeblen I'm 100% certain that my RAM, HDDs, CPU and cooling fans works. I don't need to make such test unless my PSU works on my friend's PC. (Rig : Phenom X4 9950 BE, 4GB DDR2 RAM, GeForce 9800GT + 2 HDDs)

If all else fail, I consult a PC shop. No real choice.