If you choose to install Windows on the same drive where an existing installation is already present, the setup program creates a folder named Windows.old in the root of the drive. This folder is used to store a full backup of the previously installed OS, including the boot manager and installed apps. This is very handy if you plan to uninstall the currently installed Windows version and return to the previously installed release. It can also be handy if you forgot to transfer some files or settings to the new installation. However, if you are already finished with the migration, then Windows.old just wastes your disk space for no reason. Here is how you can delete it and why it is better to use the built-in OS tools.
- Press Win + R keys together on the keyboard and type the following
- In the search results, cleanmgr will be listed. Right click it and choose "Run as administrator". When it starts, wait till the disk space calculation is finished.
- Scroll down to the end of the options list and locate the Previous Windows installation(s) option. Tick it and click the OK button.
That's it. You can of course also delete your C:\Windows.old folder manually, but the Disk Cleanup application is more useful, because it will resolve NTFS access rights automatically and delete all files which are not accessible from your user account. In case of manual deletion, you will have to resolve those access rights by yourself, otherwise you will not be able to delete the C:\Windows.old folder.