incorrect logon dialog box

We have just upgraded from WinNt4 to Win2000 on our servers, and now several of my Win98 clients get the Windows Logon box instead of the Microsoft Networking. If they go ahead and logon, and then logoff they can get to the Microsoft Networking dialog box. After that, the logon goes as expected. This does not happen on all of the Win98 PC’s yet all of them are getting their network setttngs through the same dhcp server. Has anyone else out there run into this? If so what did you do?