When connecting a PC to an Essentials 2012 server using the Connect wizard the other day we ran into the good old message "Another installation is in progress" - we checked for any msiexec.exe processes running and there were none. The next not so obvious place to look was the registry. I had a feeling that it could be the PendingFileRename keys I've run into a few times before, usually with Symantec Endpoint installations.
Navigate to the below keys and delete the PendingFileRename keys you find.
Always backup your registry before doing anything with it, even if you know what you're doing!