It doesn't sound like the Default Delivery Location has been changed.
If the Personal Folders containing the stuck messages is no longer the Default Delivery Location, Outlook 2003 will stop trying to send from that particular Outbox. It will be using the Outbox for the Personal Folders that
is the Default Delivery Location.
If you right click the Personal Folders containing the stuck message, is the option:
Close "Personal Folders enabled?
If that option is not enabled or "greyed" out, then that Personal Folders is still the Default Delivery Location.
Is Outlook 2003 still in Work Offline mode? Closing and restarting Outlook 2003 in Work Offline mode should have also made Outlook stop trying to send from the Outbox.