Try a repair installation.
Do you have the full XP CD? If so, boot from it. Do not choose the first "R" for repair. Continue. After the license (F8), you should see your XP installation listed. Follow those directions and press "R" then.
Do you have the full XP CD? If so, boot from it. Do not choose the first "R" for repair. Continue. After the license (F8), you should see your XP installation listed. Follow those directions and press "R" then.