Outlook XP won't authenticate

Exchange 2000 server. Outlook Express will connect using IMAP or POP3 but Outlook XP will not connect when I select "connect to Microsoft Exchange Server" when configuring an email account. If I've created a mailbox in AD Users & Computers for a user, and I have basically a default install of Exchange... what are some things I can check to get Outlook XP to work with the "Microsoft Exchange Server" option. Permissions on the mailbox in exchange are default. When I launch Outlook XP it prompts me for a username, pass, and domain name. I log into the domain fine so why is it asking me for this info?