Just to let you know if you're interested, a friend managed to reformat and reinstall XP for me - completely successfully.
Looked over his shoulder as he was doing it and it isn't that complicated generally - although he did run into a few hiccups which he overcame but in fairness, I wouldn't have known what to do.
Bit of a pain time-wise and updating Windows, finding drivers etc (especially on dial-up) but would probably attempt it myself if I ever needed to again.