1. Have you made any changes to the computer recently?
2. When exactly are you getting this message?
3. What is the complete error message or error code?
I would suggest you to perform a SFC scan and check if it helps.
Description of Windows XP and Windows Server 2003 System File Checker (Sfc.exe):http://support.microsoft.com/kb/310747
Note: You requires Windows XP disc to perform a SFC scan.