Power supply or hard drive?

[Nite Owl]

I've been having a transient problem with my desktop snapping off. I don't know how else to describe it, it just suddenly shuts down. It started a few months ago but it was rare but in June it started to happen more frequently. It started freezing too. I've called Dell a number of times and it always just started up and I was on my way until it happened again. My son thinks it might be the power supply, Dell says the hard drive. I was on the phone between the hardware and software techs today for about 5hrs and they are sending someone to install a new hard drive on Monday. I have things backed up but don't really want to replace the hard drive unless I really have to.

Today the fan was sort of noisy and I decided to shut down and restart, it didn't shut off by itself this time. When I restarted it would only go as far as the black screen with the Windows flag and the little blue squares would stop and that was it. Did system restore and various diagnostics for the hardware which were normal.

The tech tried a new installation of Windows this afternoon and it wouldn't install. When it got to the 'do you agree' it wouldn't accept and go on.

It's a Dell 8250, P4, 2.4,windows xp pro sp1, 2 1/2 yrs old. When it works it worked well, not slow at all.

Any ideas of which it could be?