Please, I've struggled 2 days to fix it or at least find whats causing this issue (came down to MB or CPU)
MB - ASUS P5B LGA 775 Intel P965 Express ATX Intel Motherboard
CPU - e4400 Intel dual core
I've had this built overclocked stable at 2.8 ghz with OCZ 800mhz Platinum Rev2 at 2.05v running smooth for 2 and a half years on a 480w PSU with x1950 pro gpu.
2 days ago when I booted the PC I start hearing odd, struggling sound from the video cards fan. Like its lacking energy to get a steady power going. The red and green lights on the case as well as the keyboard lights kept flashing. After like 30 sec the PC then shut down or restarted itself and going back in same state. Monitor light is orange.
I removed every single piece. I used a different PSU as well to see if thats the fault, but same thing. I only had the PSU, CPU and MB attached and the red light was solid, but green light kept flashing. Once I removed the CPU cover and rebooted, there wasn't any green light. When I slowly was moving or even touching the cpu cover that green flashing light kept coming. Its like its so sensitive. I cannot boot the PC without the CPU cover and I only managed to get a green steady light once or twice for like 30 seconds, but that wasn't enough to get to a proper boot. Any opinons, thoughts? Please, oh and prior to that day I also couldn't boot the machine. Turned the psu power button off and on and the BIOS was set on default, but I still managed to have everything working all day without issues.
My first suspect is practically always the PSU. See if you can test your PSU in another working system. If you cannot do that, try to borrow a DMM to measure the voltages. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead.
You can repeat the voltage checks with the PSU connected to the motherboard.
PSU would've been my guess, but it sounds like you already tried a different one, so unless you're extremely unlucky, that's not it.
I wonder if something came loose and the motherboard is getting shorted against the case. That could cause a problem like this. You can check for that by removing the guts of the machine from the case and trying to boot it briefly that way.