We have a number of SBS 2003 sites where we created a new domain administrator account with identical permissions to the built-in 500 account.
Microsoft Windows Small Business Server Setup
Setup has detected the following requirements have not been met: You must be logged on using the built-in administrator account.
The above error message came up when we were running the Add/Remove Programs process to uninstall Exchange 2003 on the source server.
So, we enabled the built-in 500 domain admin account, reset the password, logged off, logged on, and re-ran the Add/Remove process.
We were then able to move on with removing Exchange 2003.
NOTE: It is important to have the Exchange 2003 install directory (exchsrvr65) or CD 2 from the SBS 2003 setup disks handy!
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book