PC won't switch on

[Mattk]

My pc suddenly switched off on Sunday and now won't turn back on.I have changed power leads,PSU's and even plug sockets to no avail.When the power button is pressed the motherboard power light comes on and when the power button is held in it turns off again so I know power and the power switch arent the issue.Last night when blindly trying to find the problem after numerous turnings on and off again it blinked into life and got as far as loading up the desktop before again shutting off.
I fear the problem may be serious but does anyone have any ideas exactly what it might be ?

Thx for any clues !

[jake-the-peg]

Well the good news if you got the desktop up again is that your hard disk seems OK.

Normally I'd say this sounds like the PC's PSU but you say you replaced this and it's OK.

Could it be an overheating problem? Is the CPU fan still working OK? are any of the heat sinks loose?

[Mattk]

Thx for the suggestion.
Everything was whirring away fine as far as I could tell when it did boot up but I will double check the cpu fan.
Would the cpu fan not working stop the entire pc from starting tho ? Wouldnt the pc start as normal and then once heat became an issue just shut down ?

[jake-the-peg]

It rather depends on the PC.

You're sure the PC CPU you tried was good? It still sounds a bit like a power supply, they often fail with enough power just to light the LEDs and maybe to turn the fan a bit..

The other thing to do is to remove all cards, memory and connections one at a time and reseat them.

[fo3nix]

mattk: yes, but assuming you have a fairly new CPU, this can be a pretty short period of time.

Was it all working fine prior to it shutting down? Did it shut down 'properly', with windows going through the stages, or did it literally just turn off?

[Space]

what exactly do you call " PSU's" ?

[Mattk]

thx guys.
the problem first showed itself a few weeks ago in a peculiar way.i normally am plugged into a surge protected extension lead and some mornings the pc wouldnt turn on.I would move the pc plug to the wall socket and the pc would then start fine.i would then move the plug back to the extension lead and again it would start fine.this would happen once every few weeks until this week !
I have tried 2 psu's(power supply unit's - spaced) where both would appear to be fine.I will try the suggestion of removing all additional components and starting up and I still have my old cpu so maybe I will pop that back in also and see if that makes a difference too.

[jake-the-peg]

Sorry meant PSU not CPU - acronym overload