You will need to do a repair install. Follow these steps:
Start the operating system from the CD-ROM,
When the computer starts from the CD, the system checks your hardware
and then prompts you to select one of the following options:
To set up 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.
Press ENTER.
Press F8 to accept the Licensing Agreement.
A box lists your current Windows XP installation, and then the system
prompts you to select one of the following options:
To repair the selected Windows XP installation, press R.
To continue installing a fresh copy of Windows XP without repairing, press
ESC.
Press R to start the automatic repair process. After repairing Windows XP,
you will probably need re-download all updates, starting with SP1. You can
go to the Windows site and download
the XP Rollup 1 package, which will make the reinstalling of the updates
much easier and quicker.
The repair installation should leave all your data and settings intact, but
you may want to backup critical data.