Boot from your Windows Install CD and choose Recovery Console.
Once at the DOS prompt run:
This will scan your computer for available OSes to boot from. When it asks if you want to add any installations, enter "Y" for yes.
Then it will ask you for a name for this OS (Load Identifier), just call it Windows XP Home or Professional, depending on what you have.
Next you will have a prompt asking for loading options. Just enter "/Fastdetect" and press Enter.
Reboot.