I get this message when I try to uninstall Exchange 2003 I get this error message:
“One or more users currently use a mailbox store on this server”
Here is how to find the problematic mailboxes.
http://support.microsoft.com/?kbid=279202
Here is another article on how to completely uninstall Exchange 2003
http://www.msexchange.org/tutorials/Remove-Exchange-server-entire-Exchange-organization.html