What is the Windows.old Folder and How Do You Delete It?
This folder is created when you upgrade from one version of Windows to another. The Windows.old folder actually contains all the files and data from your previous Windows installation.
Yes you can delete it but make sure you have all the files and your system is working as expected.
To delete it launch diskclean up wizard type cleanmgr on run and hit enter now choose the drive where you have installed OS and hit ok.
select previous windows installation and click Ok to delete.