Windows 2000 Pro.

I have a user who somehow corrupted a dll that is used by the logon application in windows2000 pro. It is called an msgina.dll. When I boot the system it gives me an error message about the msgina.dll being corrupted and to replace the file and the only option I have is to reboot. I need to be able to get into the system and copy the file and register it. So if anyone knows of a way to boot into the system to replace the file and register it. I thank you in advance.