You need to run a Repair of XP to fix this.

You will need your XP installation CD for this, so I hope you have one.

 

A repair won't lose any data, other than Service Pack (if not on CD), any Windows Updates and unsigned drivers, which will need to be re-installed afterwards.
Stay offline to do a repair and do not go online again without a firewall enabled. After repairing XP, install Service Pack (if necessary), firewall and then you must install all Critical Updates next.

 

How to Repair XP.
1. Boot the computer from your Windows XP CD.

2. When you see the "Welcome To Setup" screen press [Enter] to start the Windows Setup.

Note: Do not choose "To repair a Windows XP installation using the Recovery Console, press R", you Do Not want to load Recovery Console.

3. Accept the Licence Agreement and Windows will search for existing Windows installations.

4. Select the XP installation you want to repair from the list and press to start the repair. Setup will copy the necessary files to the hard drive and reboot.

5. Do not press any key to boot from CD when the message appears. Setup will then continue as if it were doing a clean install, but your applications and settings will remain intact.

http://www.theeldergeek.com/repairing_windows_xp.htm
Scroll way down page to
Windows Installation CD - Repair Current Installation

 

http://michaelstevenstech.com/XPrepairinstall.htm

 

 

If the "Repair" option is not availabe, just quit otherwise you will do a clean install of XP