Performing a repair allows you to try and fix your existing installation of Windows XP. The main advantage of doing this is that you do not have to reinstall any applications, restore any data, or reconfigure your personalized settings.
The basic steps for performing a repair are as follows:

1. Boot your computer using the Windows XP CD.

2. When prompted press Enter to install Windows XP.
3. After pressing F8 to accept the End-Use License Agreement, setup should detect your existing Windows installation.
4. Press R to begin the recovery process.
5. Setup will copy the necessary files to your hard drive and then reboot.
6. Do not press any key to boot from the CD-ROM this time. Instead let setup continue.

After the repair is complete, be sure to reinstall the latest service packs and updates since they will have been removed during the repair process.