After an in-place upgrade in the latest versions of Windows (2008 and up), the installation keeps a complete copy of the previous operating system into a folder named “Windows.Old”, in case you’d like to revert the upgrade. This is a useful option, but it leave your installation with a huge wasted space on the disk.
If you simply try to delete the folder from the file explorer, you will find out it is locked. In a previous article titled Reclaim disk space after an in-place upgrade to Windows Server 2012, I explained how it was possible to remove this folder by installing the Disk Cleanup feature. However, this method requires a reboot, so I was looking for a better solution, and I found a quicker method that does not requires a reboot.
Open a command prompt with administrative privileges, and run this three commands:
takeown /F c:\Windows.old\* /R /A /D Y cacls c:\Windows.old\*.* /T /grant administrators:F rmdir /S /Q c:\Windows.old
The old directory is gone, and you have reclaimed a large amount of disk space.