|Get your XP CD and put it in the CD-ROM and reboot.|
Ensure you have the CD-ROM selected as the first boot device (in the BIOS) and boot to it.
Start the install sequence, skip past the "recovery console" and when you get to the point where it sees your existing XP installation you'll be given the option to "Repair" that install. Choose that option.
When the whole repair is done, you will have to hit the Windows Update site and redo most, if not all, updates and security fixes.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.