Unconfigured Named MSSQL Instance

If your installation errors out half way through, you may wind up with an unconfigured named instance. Short of uninstalling all of Microsoft SQL, you'll have to manually remove the instance in order to re-use the name.

Use regedit to find the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server


Under Instance Names\SQL, delete the value corresponding to your unconfigured instance. Now look for the corresponding MSSQLxx_xx.InstanceName key and delete it.

You will also want to delete the corresponding instance folder, the default path would be C:\Program Files\Microsoft SQL Server\MSSQLxx_xx.InstanceName.