Exchange 2003 removal in a Exchange 2010 network
I get this error
The error that you are receiving occurs because the Exchange Server 2003 Setup program detects during the uninstallation that the Active Directory schema has already been extended.
But then, since the CN=Active Directory Connections container is not available in the Active Directory, then that means the installation of Exchange Server 2003 was not successful.
My recommendation here will be that you should not run the Exchange Server 2003 Setup /forestprep command in an environment that is purely Exchange Server 2007.
You may find that command appears to successfully create the CN=Active Directory Connections container, but when it comes to the installation of Exchange Server 2003 you will find that it will still be unsuccessful and therefore leading to the error message that you are getting.