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.