I think you have to re-install xp again. If you have the XP cd you could try using the repair option first. XP will boot from the install cd.
If you do not have a copy of XP you could borrow one and use your licence number when you install again. The cd's are the same it is the Licence you have paid for that is important, do not use someone else's number it will not work when you try to activate XP. It is one number per pc only.