Outlook is asking for Server 2K credentials

I have found that this happens only on laptops and only with XP. The Outlook
2K client asks for the user name, password, and domain name even though the
network sign on was successful. The users, of course, is unable to get his
email no matter what sign on he puts in. I have already hacked his registry,
imported was Microsoft said was the failing key from a working laptop. All
to no avail. He cannot get outlook. Can anyone help??? Please!