Why won't my old PC boot? It has power...?

[quemans]

I was wondering if anyone had any idea why my HP PC won't boot.
It used to have to "warm up" before booting up.
I would plug it in and flip the toggle switch on the back of the computer "down". Let it run for about 10 minutes then switch the computer off. Flip the toggle switch on the back of the power supply "up" and it would boot properly.
Warm temperature seems to have an effect.
If I place a space heater near the computer it will boot in a shorter amount of time.
Also, it will only boot when plugged into certain outlets in the house and if nothing else is plugged in near it.
The problem is now the computer won't boot up at all. It has power but won't start up.
Thanks for any help or suggestions.

[cunny]

All points towards PSU fault, apart from last sentence.

If there is power on the mainboard, a orangle LED should be lit. Check for that. If there is no power on board PSU fault. If there IS then check for loose connections

[Digital Twiddler]

Also, some motherboards had capacitors that used expand and break. Evident if you remove the case and look around the processor area, There will be some small 'can like' components and their tops will have bulged open and split - these will need replacing.....