Exchange problem after user upgrade

Has anyone else experienced problems with Exchange mailboxes after the user changes from a W98 machine to WinXP?
We have one server running Small Business Server 2000 and Exchange 2000. Outlook 2000 client. Recently two different users have gone directly from W98 to XP and they both have the same problem. They can receive email okay, but when they try to send or access the global address list they get the error message “Network problems are preventing connection to the Microsoft Exchange Server computer”. If they delete and re-create the user profile and then open Outlook, this problem doesn’t occur, but after shutting down Outlook and re-opening it the problem re-occurs.

Any suggestions?
Thanks
anseris