Could you please describe exactly what the computer is doing. Does the Windows XP "splash" appear (Should say Windows XP in the middle of the screen with a progress bar below)? If you don't see a splash screen, here's what to do:

1. Insert your Windows XP Setup/Recovery CD (preferably setup)
2. When you are prompted whether to enter setup or run recovery press R.

3. Press C
4. You may be asked to login, do so (it will be a text prompt).
5. Type C:, then type CD \,
6. Type fixmbr
7. Type fixboot
8. Reboot and remove the CD