Microsoft Knowledge Base Article 323042 contains the following summary:

When you try to upgrade a Windows 2000 Server computer to Windows Server 2003, you may receive the following message:

You must be an Administrator to run this application.

NOTE: You may receive this message even if you are logged on to the server as Administrator:

This behavior may occur if the administrative rights that you need are either not defined or have been disabled because of a policy setting on the local computer or in the domain.

This article summarizes the user rights that you need to upgrade a computer from Windows 2000 to Windows Server 2003 Standard Edition or to Windows Server 2003 Enterprise Edition.