This is usually caused when a user has previously set their Outlook client to store the password and the password and since been changed. This results in Outlook constantly trying to login using the old stored credentials.
The fix for Outlook 2007:
1. Select Tools -> Account Settings -> Then select the Exchange account that Outlook is prompting the login credentials for.
2. From the Account Settings window click on More Settings -> Security -> Then check the box under User Identification "Always prompt for logon credentials."
The fix for Outlook 2010:
1. Select File -> Account Settings -> Accounts Settings -> Then select the Exchange account that Outlook is prompting the login credentials for and click Change.
2. Click on More Settings.
3. Select the Security tab and check the box next to Always prompt for logon credentials.
4. Click Ok, then on the Accounts Settings window, click on Next then Finish.
From this point, Outlook should only prompt you on the initial launch of the program for your login information.