Due to licensing problem, I only can upgrade MS Outlook 2000 to 2003
version, I have a few new computer that is installed with MS Office
2003, I faced the same problem too. No problem during the installation.

During the Upgrading installation, the system will prompt to upgrade,
just press the "OK"s button will do, nothing much.

I found that if the Mail applet in the Control panel disappear(power
user or lower), you cannot use the office. Once you have the
administrator status, the Mail applet will appear again.