Hers how to do a Repair install:
Configure your computer to start from the CD-ROM drive. Then insert your Windows XP Setup CD, and restart your computer.
1.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.
2.Press ENTER when you see the message To setup Windows XP now, and then press ENTER displayed on the Welcome to Setup screen.
3.Do not choose the option to press R to use the Recovery Console.
4.In the Windows XP Licensing Agreement, press F8 to agree to the license agreement.
5.Make sure that your current installation of Windows XP is selected in the box, and then press R to repair Windows XP.
6.Follow the instructions on the screen to complete Setup.
If you have xp sp2 installed it needs to be incorporated into your xp disk or it will not offer the repair option only the clean install.
Slipstreaming Windows XP Service Pack 2 and Create Bootable CD