Computer unresponsive before windows account screen

[Hathaway]

Running Windows XP SP2. I'm not sure if the problem is windows or not, so please move this as you see fit.

My computer will no longer get as far as even the Windows account screen before it seems to go into some sort of idle mode. It'll run fine at start up, go through the usual procedure running through the BIOS info, and it'll go through the Windows Loading screen fine. But as soon as that goes off, the monitor can no longer detect the computer and the computer becomes completely unresponsive. It's still running but there's nothing I can do accept force shut down via the power switch.
However it will work in safe mode fine by all accounts. I've tried a complete reformat using my computer manufacturer's disc but this hasn't had any effect.

Can anyone suggest what might be causing this or what I might be able to do to solve it?
If there's any info I can provide to help diagnose the problem please let me know how I can obtain it for you.
Thank you.

[Hathaway]

Just to add, out of curiosity I let it run instead of switching it off and after about 15-20 minutes it restarted itself without any input from me. It got slightly further this time, showing the blue screen the accounts come up on, but again became unresponsive.

[Hathaway]

I'm toying with the idea of buying a new copy of Windows XP and a new hard drive, but I don't want to spend money and find that's not the cause. What do you think?

[Hathaway]

I don't know if there are rules against bumping, I appologise if so. But I get the feeling this message will slip into obscurity if I do not, and I could really do with any assistance.