Windows.old is a folder that contains your previous Windows installation. It is created automatically when you upgrade from one version of Windows to another (for example, from Windows 10 to Windows 11, or from one Windows 11 update to the next).
This folder can be very large -- often 10 GB to 30 GB or more -- because it contains a complete copy of your previous operating system. Windows keeps it so you can roll back to the previous version if something goes wrong with the upgrade.
RJL System Cleaner rates Windows.old as Review and shows a specific warning that is separate from the standard Review gate:
"Deleting this will permanently prevent you from rolling back to your previous version of Windows. This cannot be undone."
Once deleted, there is no way to recover the previous Windows installation. If your computer is working well after the upgrade and you do not plan to roll back, it is safe to delete. But if you are unsure, keep it -- Windows automatically removes it after approximately 10 days anyway.
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