What we tried 1st:
A repair & re-install but to no avail, we even repaired the pst with a scanpst.exe. When trying to open a fresh install of outlook 2003 we would get - Error message when you start Outlook 2003: "Unable to open your default e-mail folders", even with trying to open outlook.exe /safe or with no mail support.
I started to look over his email settings again and realized that with the AOL(AOHELL) he had setup to receive into his Outlook, it used imap settings so I killed that imap email all together and voilà !
After all that we found the issue to be with the aol imap account. If you are using imap then kill it, if that doesn't help, maybe some other things suggested here will help you.
Computer Repair Lakeland, FL