Obtain a bootable Windows XP CD, and use it to boot.
Wait through all the Bill-messages until you get the first prompt. Choose R to repair an existing installation.
It will search and prompt the Windows installation, showing :
choose 1, and it will ask for the Admin password. If you have one enter it or just press Enter.
C:\WINDOWS> CD ..
C:\> FIXBOOT C:
C:\> BOOTCFG /rebuild
After the BOOTCFG, it will ask if you want to add the Installation it found, and to be safe answer “Y”.