This should be in the helproom.
Do you have an XP CD?
Repair XP by install over the top of existing system
1. Boot the computer using the XP CD. You may need to change the boot order in the system BIOS so the CD boots before the hard drive. Check your system documentation for steps to access the BIOS and change the boot order.
2. When you see the "Welcome To Setup" screen, you will see This portion of the Setup program prepares Microsoft Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
3. Press Enter to start the Windows Setup.
To setup Windows XP now and Repair Install , press ENTER.
DO NOT choose "To repair a Windows XP installation using the Recovery Console, press R", (you Do Not want to load Recovery Console).
4. Accept the License Agreement and Windows will search for existing Windows installations.
5. Select the XP installation you want to repair from the list and press R to start the repair.
6. Setup will copy the necessary files to the hard drive and reboot.
Do not press any key to boot from CD when the message appears.
Setup will continue as if it were doing a clean install, but your applications and settings will remain intact.