Its not quite as straught forward as sinoj suggests due to the new vista boot loader and its not without ist dangers as a slip up may make your system unusable and you may have to re-install Windows.
That said the proocedure is as follows:-
1. Backup: Just in case !!
2. Reboot and Boot to your XP CD-ROM
3. Start the Recovery Console
4. Run Fixboot
5. Run fixmbr to reset the master boot record
6. Exit the Recovery Console
7. Run bootcfg /rebuild
8. Reboot