With a SSD you don’t have space to waste, so I was searching how to delete the c:\windows.old folder, here is what I have found.
The easiest and probably supported way is to do so from the Disk Clean up prompt. To do so go to your Computer, right click on C and select properties. From there click on Disk Cleanup.
Click on Clean up system files. This will change the options in the “Files to delete:” section and will include “Older version of Windows”. Select that option as well as any other option you wish and press OK. This will get rid of the Windows.old directory without any errors.