I had the same problem when I had to reinstall my windows XP from a CD too. I reinstalled it on the same PC, same CD and everything and when I activated it online it said invalid serial number so I had to call microsoft and they did it for me and all was good. I don't know why this happened and it's kinda odd to me why it did happen. My guess is that they might have had a problem with their activation or changed something.... who knows????