ntdll.dll error

I am running Windows 2000 and Office XP on my PC. When I go to open Excel, PowerPoint, Outlook, or Project I receive the following error:

The Procedure entry point LdrLockLoaderLock could not be located in the dynamic link library ntdll.dll.

After closing the error message I can continue using the application but the messages are getting annoying.

Any suggestions? I have tried replacing the file with another one from another PC but the system tells me the file is in use and cannot replace it.