If you participated in the Windows Insider preview program for Windows 10, you likely will find a Windows.old folder on the root of your hard drive from old preview builds. This folder can take up a significant amount of space (12 GB on my PC) and is no longer needed after you have upgraded to a new build, such as the final RTM build 10240. If you are looking to free up disk space, cleaning this folder is the perfect candidate. Unfortunately, deleting it is more complicated than deleting a normal folder.
Trying to delete the Windows.old folder from File Explorer will result in a lot of permission denied errors. This is a special folder that can only be deleted from the disk cleanup application.
1. Click on the Start Button and type in Disk Cleanup and hit Enter.
2. When the Disk Cleanup utility has loaded, click on the Clean up system files button under the Description section.
3. After the utility scans your computer, scroll through the list and check the box next to Previous Windows installation(s).
4. Click OK, and then click Delete Files on the confirmation screen to proceed.
5. As the Disk Cleanup utility begins processing, you will be prompted once again before the old Windows installation files are deleted. Click yes when prompted.
The delete process will take some time but after it is completed, the Disk Cleanup utility will close and files in the Windows.old folder will be removed freeing up a significant amount of disk space.