Now I might be able to recover from this but I would use my full retail XP SP2 CD to try a "XP REPAIR INSTALL" (see google) or a parallel install then reach over to the other OS to edit it's registry.
This is fairly advanced stuff so I'm going to share that most will just wipe out and start over. There is a CHEAP EXIT however. Remove this hard disk, put in a blank and install your OS there. Now add the old drive as a slave and copy your files over.