When your Windows 7 or 8.1 machine is upgraded to Windows 10 your Windows folder from the previous OS is stored on your hard drive in a folder named Windows.old. The files in this folder are used to revert back to your old OS if there is any issue in Windows 10 during the upgrade. If the upgrade goes well the Windows.old folder it is maintained so that you can revert back during the first 30 days after upgrading to Windows 10. I suspect that after the 30 day period this folder will be removed automatically or you could possibly be prompted to delete it. One thing is for sure – there is only a 30 day window to revert back to your previous OS. After that reverting back means reinstallation of your former version of Windows.