All it should take is to insert the WindowsXP install CD and allow everything to load just as if you will install a clean system. The preparation will check the partition for a pre-existing operating system, it will find your operating system that is already there, and it will then ask if you wish to repair this system, install over this system, or format the drive and install a new clean operating system. I think you are not reaching this part of the normal installation to see this choice.
Just go through the normal installation routine except it will detect your system and ask you if you wish to repair it or not.
Just go through the normal installation routine except it will detect your system and ask you if you wish to repair it or not.