This folder is created when you upgrade from one version of Windows to another, starting with Windows Vista. The Windows.old folder contains all the files and data from your previous Windows installation. You can use it to restore your system to the old version of Windows if you don’t like the new version. If you’re looking for a specific file that wasn’t properly copied to your new Windows installation, you also could dig into the Windows.old folder and find it.

Essentially, the Windows.old folder just contains the old Windows system. From the Windows system files to your installed programs and each user account’s settings and files, it’s all here. The new version of Windows just keeps it around in case you’d like to go back to that older version of Windows or in case you need to dig in and find a file.

But, don’t wait too long—Windows will automatically delete the Windows.old folder to free up space after a month.