Cannot login to Email Using Outlook

I have a user that cannot login to his Email account unsing Outlook 2000. He is connecting to our exchange server. When he tries to open Outlook, It asks for name – password and Domain. I can change to my mailbox in the settings and it will open. He can open his mail on another pc. He can also open OWA on this pc and it works. I have reloaded OFFICE and deleted his mail profile on his pc. Same problem. Any suggestions?