If you choose to install Windows 10 on the same drive where an existing installation is already present, the setup program will create a folder named Windows.old in the root of the drive. This folder will contain a full backup of a previous installation of Windows 8 or Windows 7, 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 useful 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 should delete it and why it is better to use the built-in OS tools instead of simply deleting the folder from Explorer.
Update: Starting with Windows 10 Fall Creators Update, you can use the Storage Sense feature to delete the Windows.old folder. Check out the following article:
- 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 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.