Cannot solve Outlook error: "The messaging interface has returned an unknown error. If the problem persist, restart Outlook."

I am having problems performing several task in Outlook. Emails are sending
but remain in the outbox and resend again and again. When I try to delete
emails from the outbox, I get the following error: "The messaging interface
has returned an unknown error. If the problem persist, restart Outlook." I
found a KB article that refers to installing an update to solve this
problem. When installing the update found at
http://support.microsoft.com/kb/300551/EN-US/, it ends with a failure error.
I have also gone to the Office Update site and downloaded and installed all
updates without any errors. Still the problem persist. I then tried Detect
and Repair. I tried to re-install Office. I uninstalled and re-installed
Office. Still the problem persist. I tried creating a new PST file
thinking the other might be corrupted, still the problem persist.

Does anyone know how to solve this error in Outlook 2002? "The messaging
interface has returned an unknown error. If the problem persist, restart
Outlook."

Thanks!