t “R”.

The Recovery Console will search for instances of Windows and ask you to select one (there will probably only be one). Enter the number pertaining to your XP installation, and you'll be prompted for the Administrator password (if you're not sure what it is you likely don't have one, just hit enter).

Now, enter “fixmbr” (confirm with “y”) and upon completion enter “fixboot” (confirm with “y”).

Enter “exit” to reboot your system. Remove your XP CD and you should be good to go.