User name&password prompt

When I start Outlook 2000 and click on send/receive, I
continually get prompted to enter my user name and
password for my Internet email account, even though both
the user name and password are already filled in. I've
tried entering the info manually just to be certain it's
correct and the result is the same. What is causing this
and what can I do to fix it? I am able to send mail, but I
cannot receive new mail.

Thank you