Well this is an off-topic question but...
Insert your Windows XP disk and restart your computer. You will be prompted
to press any key to boot from CD-Rom: do it.
During Windows Setup, you will get the choice to Install, Upgrade or Repair:
choose Repair. If it is only the NTLDR that is damaged, it should repair in
around 1 second and then prompt you to remove the CD and restart your
computer.