Unable to install MS Office Pro 2003 due to an invalid key error

When I run the MS Office Pro 2003 setup program, I get the following error
message:

"You have not entered a valid Product Key.

Please check the number located on the sticker on the back of the CD case or
on your Certificate of Authenticity."

The odd thing here is that I was not even prompted to enter a license key.
I searched the MS KB and one of the solutions was to uninstall all the MS
Office components via the Add/Remove Program wizard, so I did that. However,
when I tried to re-install MS Office, the same product key error is now being
spitted out.

Any ideas on what I need to do to be able to install MS Office 2003 again?

Thanks in advance!


Cheers,
Rene