IT could jsut be that when Windows XP isntalled it didnt isntall the drivers right. Then when it updated the drivers on first boot the drive was recognized as empty.
Honestly i dont know exactly how or why it happens. But this isnt the first time i have heard of Windows install that has lost drive data.
Also might want to check out the life of the drive. If it makes any funny noises or is really hot you might want to check it out. It could be getting close to failing on you.