Outlook Web Access Exchange 5.5 2K Serve

Scenario:
We are running Exchange 5.5 on a Windows 2000 server. Win98 clients w/ IE 5.5 on the network are able to access the logon screen for OWA and log on successfully. Win2000 Professional clients with IE 5.5 can access the logon screen for OWA but cannot log on. Same user account on Win98 client that accesses OWA cannot do so on Win2k Pro client.
We have several 98 and 2k clients and same is true for each.