Software

to enter logon credentials when opening Outlook can occur for a number of reasons.

The most common cause has to do with a problem with a users logon credentials such as an expired password or an account lockout. When a user logs on to a network, a copy of their credentials are used by Exchange server to authenticate that user and allow them access to mailbox resources. If a problem exists with a users credentials, Exchange will not be able to authenticate the user thus prompting for the user to re-enter their credentials.

Another common problem that can generate a prompt for logon credentials is a faulty network connection. A failed or slow connection to the server or the server not being available will also result in your problem.

I would start with these things first, then if the problem persists, begin to dig deeper.

Good luck!

Get this program. It has helped me numerous times finding various problems with user accounts getting locked out... everything from a service or program running as the user (with old credentials saved)to a domain controller that wasn't properly receiving AD account info updates...first you find which server is locking out the account and then you scour the event viewer on that server to to see if you can get any clues.
http://www.microsoft.com/downloads/details.aspx?FamilyID=D1A5ED1D-CD55-4829-A189-99515B0E90F7&displaylang=en