If you choose to install Windows 10 on the disk where an existing installation already exists, the installer will rename the old Window's folder to Windows.old in the root of your disk. This folder will contain a full backup of the previous Window 8 or Window 7 installation. The folder also includes the boot manager and your installed applications.
This feature is very convenient if you plan to uninstall your current installed version Windows and return to the previous installed version. It may also be useful if you have forgotten to transfer some files or settings to the new installation.
However, if you've finished with all of this, then the Windows.old folder only captures your disk space for no reason.
Below we describe how to delete it using built-in Window tools instead of simply deleting the folder from Explorer.
Press the two Win + R keys together and type the following command in the RUN box that will appear:
Cleanmgr will appear. Right-click on it and select "Run as Administrator". When the application opens, wait for it to finish calculating your disk space.
Scroll down, at the end of the list of options you will find the option to delete the previous Window installation. Select it and click OK.
You have finished, of course you can also manually delete the C: \ Windows.old folder, but Disk Cleanup will automatically resolve the NTFS access permissions and delete all files that are not accessible from your account.
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In the event of a manual deletion, you must resolve these permissions, otherwise you will not be able to delete all the files in the C: \ Windows.old folder.