Iwish I could help! I'm in the same trouble myself at the moment.
Mine has been caused by bad sectors on the HDD. I'm trying to recover it but I have a feeling that I'll just have to fork out for a new one.
I've had to boot with an old Win 98 boot disk, FDISK, SCANDISK and now XP is trying to format it into NTFS. There are strange noises coming from it - so I think it is heading for the HDD graveyard in the sky.
I wish you the best of luck!