I would buy a second hard disk, if I were you, and clean-install XP onto it while your old disk is removed from the PC.
Then re-attach your old HDD as a slave, copy back all the settings and data and then use the disk for storage. That assumes that you can physically add another HDD internally - the alternative is to buy a USB caddy and put it in that.
Backup critical stuff anyway, just to be sure, to be sure! Use DVD or USB external disk.
SD