After a Windows in-place upgrade (e.g. Windows 7 -> Windows 10 / Windows Server 2008R2 -> Windows Server 2012R2) the old Windows directory is stored on the system drive as a backup.
Since the Windows directory needs some space, it makes sense to delete it after the upgrade. The easiest way to do this is the following CMD command:
RD /S /Q %SystemDrive%\windows.old
Important: The CMD must be started as administrator.