I found this in a few different places around the 'net - hopefully it's helpful:

Perform a Repair Installation

How do you perform a reinstallation of Windows XP, sometimes called a repair installation?

• Configure your computer to start from the CD-ROM drive. For more information about how to do this, refer to your computer's documentation or contact your computer manufacturer.
  
• Then insert your Windows XP Setup CD, and restart your computer.
• When the Press any key to boot from CD message is displayed on your screen, press a key to start your computer from the Windows XP CD.
• Press ENTER when you see the message To setup Windows XP now, and then press ENTER displayed on the Welcome to Setup screen. Do not choose the option to press R to use the Recovery Console.
  
• In the Windows XP Licensing Agreement, press F8 to agree to the license agreement. Make sure that your current installation of Windows XP is selected in the box, and then press R to repair Windows XP.
  
• Follow the instructions on the screen to complete Setup.