Outlook authentication dialog problem

We recently moved from NT4 and Exhange 5.5 to Windows 2000 and Exchange 2000 on the servers. Our clients are still Windows 98 and will be for a least 3 more months. We’re having the following problem with Outlook 2000.

The user logs in and authenticates with the AD server, and opens outlook, either immediately or within a few seconds after connecting (they see mail) they’re presented with a dialog box to authenticate with their username, password and domain. The fields are all blank. If they hit cancel 2 or 3 times the box goes away for some time and they’re able to fully use their email. Eventually, the box returns.

I’ve tried looking through all the settings and they are all pointed to the new server. Is there a hidden setting somewhere that could be still looking at an old server?