Boot from the windows XP CD, press the “R” key in the setup in order to start the restoration console. Select your windows XP installation from the list, and enter the administrator password.
Enter the command: “FIXMBR” (without the quotes) at the input prompt and confirm the next question with a “Y” (without the quotes). Use exit to restore the computer.
I had to use this on a PC that I had corrupted the MBR with by using a boot manager.
Worked perfectly. This may have been posted before, and if so, forgive me. If not, now you know how to repair your MBR. By the way MBR stands for Master Boot Record. THis procedure kind of replaced Fdisk I think.
from : http://www.techzonez.com/forums/showthread.php?t=3975