XP running jerky and very slow - despite format

Hey chaps.

I'm trying to fix my brother's laptop. It's got XP and has been running very slowly recently, to the point where Windows beeps were taking many seconds to play, and windows were moving very slowly. CPU kept jumping to 100% load.

We formatted it. No change.
We took out the 1gb of RAM and replaced it with the original 512 stick, but this also had no change. I removed the things I thought might be causing it, like Norton (!!), which I replaced with AVG, but this doesn't seem to have worked.

What would cause this level of slowness?! What should I try?

Faulty hard Drive?

Seems to suggest your harddrive is running in PIO mode and you're doing virus scan.
Try restore BIOS to default setting?

The hard drive did cross my mind, but I can't hear it struggling to read and hav'nt had any read errors or the like.

What is PIO mode?:O_o1:

Could be overheating causing constant CPU throttling, checked the temps?