I'd appreciate any help that anyone can provide:

The problem started slowly, and gradually got worse.

When I turn my computer on, it doesn't seem to do anything - the hard drive and CD lights come on, the fans come on, but the startup process doesn't seem to happen.  Then, after 5 - 20 minutes (if I'm lucky), it may begin the startup process (ie. the Windows XP loading screen).  However, the keyboard may or may not work, and if it does, it may only work for a while.

I've tried new keyboards, MICE, and MONITORS to rule them out (everything else like printers unplugged).  I've tried it in different outlets also to rule that out.  My laptop computer hooked up to my keyboard and monitor works in the same outlet.

I took the computer in for repair, and they found nothing since it worked perfectly fine for them.

So I took it to a friends house, where it didn't work at first, then did after a while.

Still didn't work at home.

Took it to another computer repair place, where it worked fine.

Does anyone have any ideas?  Someone suggested the power supply ... but why would the computer only not work at my house and occasionally at my friends?

(Pentium IV, 1.8 Ghz, 512 MB RAM, Win XP)

Thanks in advance.
Sounds like gremlins.

Do you have anything to risk by a reformat? Something to think about.

So it works fine everywhere else, but not at your house. And that is ALWAYS the case or it is intermittent even at your house?The supply voltage at your house may well be different to that at the computer shop or your friend's house.

You PSU (Power Supply Unit inside computer) could be 'on the edge' which would magnify any mains problems.

How does it start in safe mode?Thanks ...

I'd definitely prefer not to re-image.

If the voltage is different in my house, would a new power supply unit get my computer working again?

I can't usually get my computer to even come on, let alone try to start it in safe mode ... it's like the computer doesn't want to begin the startup processCan you 'swap' your power supply just as a test?Probably not ... It would have to be replace it and take my chance, or try something elseFrom what you have said, the only possible causes can be your mains supply, your PSU or a combination of both.
Can you run your computer off a different power point using an extension lead, maybe even from your neighbours house?

A quick QUESTION?
Aside from the location was there anything different when you tried it?
Mouse, Keyboard, Monitor? Quote

I took the computer in for repair, and they found nothing since it worked perfectly fine for them.

Try running from a UPS. You really should have one.

I'm not all that confident in what the computer repair places did ... they claimed that they ran a piece of software that does diagnostics, and it found nothing.

...I'm living in the country, so it would be very difficult to run an extension cord from a neighbours house.