network connectivity

Our network is running on WIN2K servers with WIN2K exchange. Most of the client users are WIN2K Pro and a number of WIN98 users that will be upgraded soon. Just recently we have discovered a problem with the connections where the WIN98 users when logging in, will get a message saying that user is not found and yet when you hit cancel the user will still be connected to the exchange server but not to the Domain Controller. I have checked out all the lines and they all checked out. If I loginas ADMIN on the same machine it just goes in. Does anybody have a clue as to what might be causing this problem. Is it an authentication problem on the server? If someone have idea please let me know. Thanks.