Outlook 2003 repeatedly prompts for username and password

I have a few users with Outlook 2003 and Windows 7. Every time they open outlook, They repeatedly get asked for their username and password no matter how many times they are entered. Outlook goes directly into offline mode when clicking cancel. The users are able to view their mail through outlook web access. I can open their mailboxes on a different computer with outlook 2003.

I have recreated their outlook profile. I have check all the permissions on the mailbox. "Self" has all the permissions. I have moved their mailbox between exchange servers (I have two). I have tried entering the details as "domain\username" and "server\username" and just "username"

I'm using exchange 2003 and server 2003.

Edit: I have a domain. They are able to login to windows with their active directory account .

We have run into this issue with users who have clicked the "remember this password" checkbox and then changed their passwords later.

On Windows 7 check the following to see if this fixes the issue:

Control Panel / User Accounts / Manage your Credentials

Make sure no windows credentials are saved there that are referencing your email (or sharepoint) servers.