Perform a Repair Installation

[cscgal]

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.