Generally...if system files are missing, I would try running sfc /scannow or...doing a repair install.
Both are methods for replacing/inserting missing or damaged system files...but neither is guaranteed to solve problems that may exist on a given system.
The user will need a MS XP CD to run either of these efforts.
LEARN how to use SFC.EXE (system file checker) in this article! - http://www.updatexp.com/scannow-sfc.html
How to perform a repair installation of Windows XP if Internet Explorer 7 is installed - http://support.microsoft.com/?kbid=917964
How to Perform a Windows XP Repair Install (Stevens) - http://www.michaelstevenstech.com/XPrepairinstall.htm
Depending on how long the problem has existed...and if the user is using System Restore...one can always try to go back to a restore point prior to the initial appearance of the problem. This probably ought to be tried before any other options, IMO.
Edit: Try the link USASMA posted first, looks like it directly addresses the problem.
Have you had any known malware problems/situations?
Edited by hamluis, 08 January 2009 - 09:31 AM.