Computer doesn't turn on, motherboard or CPU?

This morning my computer wouldn't turn on. Pressing the power button has no affect. When I turn off the switch at the back and push the power button after turning it back on it seems for about half a second that everything works: fans appear to start moving and the PSU light turns on.

This happens only for a brief moment (less than a second) and then everything turns back off. I can repeat this by turning off and on the switch at the back of the case or by unplugging the power cable out and plugging in again.

There is a small LED on the motherboard which is always on when power is connected (doesn't matter if the computer is on or off) and it is still on now.

I am trying to diagnose what is the cause of the problem PSU or motherboard. Any ideas?

Unplug everything you can possibly unplug, internally and externally, and remove all expansion cards and RAM. So, with nothing but the main ATX power header and CPU power header (and front panel and internal speaker connections attached - see comments) attached and no RAM, if the box STILL won't power on, you probably either have a power supply or motherboard problem (the odds of a failed CPU are nil unless you've been overclocking like crazy, and if you're savvy enough to do that you shouldn't be asking this question).

If it beeps at you, which it should if you try to power it on with no RAM, reattach stuff piece by piece (starting with the RAM) to see if you can isolate a problem.

It could be one of many components, but it is likely to be the RAM/CPU/PSU/Motherboard (in no particular order).

If you have access to some spare components or a someone else's similar machine, you can possibly isolate the problem yourself. The first task is to test one stick of RAM in another machine. If the RAM is good, leave it (on it's own) in your machine, leave the CPU and the PSU connected, but remove all other RAM/cards/disks/peripherals. If this allows you PC to at least 'turn-over', your RAM may well be the problem.

Likewise, if the problem remains, you need to either test each of the other main components either by proving them in another machine, or by replacing them with a spare one. Be sure to use compatible components for testing - that is, if you try to cannibalise another machine to test this one, make sure that they use similar components. The specification in your user guide should indicate this, or otherwise seek specific information from your OEM.

For your average home or business user, this might prove difficult, which is where your local independent repair shop comes in. Yes, you'll have to pay for labour on top of the replacement parts, but unless you have your own resources, you have no choice.

If you aren't confident under the bonnet of your PC, this might also be the safest option.