Before the computer boots up, press DEL to enter the BIOS settings. Change the first boot device to CD ROM. Then insert the Windows XP setup disk and restart.
When prompted, press any key to boot from the CD. In the setup menu, press R to repair Windows. Log on to your existing installation by pressing 1 and then pressing enter. If you have an administrator password, enter it. Type the following lines.
copy f:\i386\ntldr c:\
copy f:\i386\ntdetect.com c:\
Here, replace f:\ with the drive letter of your CD drive. Replace c:\ with the partition on which Windows is installed.
After these files have been copied, remove the CD from the drive and reboot.