When you open Control Panel, you receive a message similar to:

Windows cannot find C:\Windows\Rundll32.exe, make sure you typed the name correctly and try again.

You may have a virus or Rundll32.exe is damaged or missing.

To restore a clean version of Rundll32.exe, follow the next steps:

1. Mount the Windows XP CD-ROM.
    
2. Open a cmd.exe session.
    
3. Type expand <CD-ROM:>\i386\rundll32.ex_ %Systemroot%\rundll32.exe and press Enter.
    
4. The modifications you made will be in effect after you reboot your PC.