Oulook2000/Exchange2000 Login Window

My network is running Exchange 2000 and all clients connect with Outlook 2000. For some reason a couple people have started getting a log in diaglog box when opening Outlook. It is asking for username, password and domain. Regardless of what info is entered it will not open Outlook. We are not using internet mail. We have tried everything to get this popup to stop. Anyone have any suggestions?