The Windows.old folder is automatically created by Microsoft Windows during a major version upgrade or reinstallation, serving as an archive of the previous OS installation-including programs, user files, and system components-to enable easy rollback if issues arise with the new version. According to a recent Neowin guide published on February 13, 2026, this folder can consume substantial disk space (e.g., around 85GB in some cases) on the system drive, often prompting users to consider removal once the upgrade proves stable. While Windows typically deletes it automatically after a short period (around 10-30 days depending on the version), manual deletion is safe and irreversible if you no longer need the rollback option. The recommended method is using the built-in Disk Cleanup tool: search for it, select "Clean up system files," check "Previous Windows installation(s)," and confirm to reclaim the space without harming your current setup. Always ensure your new Windows installation is reliable before proceeding, as deleting Windows.old eliminates the ability to revert to the prior version.