If you are getting an error message with Error code 3221685466 while you are trying to uninstall Exchange 2007 SP1 Mailbox role from Windows Server 2008 R2, you may try the small tip described here:
"An error occurred. The error code was 3221685466. The message was The service is already registered.."
Solution: Change registry key value from Uninstall to Install located at
Try to uninstall again.