After upgrading to Windows 10, you may want to delete the old Windows directory (Windows.old), but when you try it tells you that you can’t. To delete the Windows.old directory you have to be a system administrator and delete the directory from the built in Disk Cleanup utility.
To delete Windows.old using the Disk Cleanup utility:
Launch the utility from the start menu by typing Free Up Disk Space and selecting the utility
Click Ok (assuming you installed to the C: drive
Click Clean up system files
Check Previous Windows installations
Click Delete Files
The Disk Cleanup utility will then delete the Windows.old directory for you.