Whenever you perform a major system upgrade (such as upgrading from Windows 10 to Windows 11, or installing a massive annual feature update like the 24H2 release), Microsoft creates a failsafe. The entire previous version of your operating system is compressed and stuffed into a folder named Windows.old located directly on your C: drive. This folder usually consumes between 15 GB and 30 GB of storage space.
This folder exists solely so that you can click a “Go back to previous version” button in the Settings menu if the new update breaks your PC. If you attempt to delete the Windows.old folder manually by pressing the Delete key, Windows will completely block you with “TrustedInstaller” permission errors. To safely delete it, you must use the official Windows storage settings.
How to Safely Delete the Windows.old Folder
You must use the Storage Sense menu in Windows 11 to safely purge the previous installation files.
- Click the Start button and select the gear icon to open Settings.
- In the left-hand navigation sidebar, ensure you are on the System tab.
- In the main window, scroll down and click on Storage.
- Wait a few seconds for Windows to calculate how your disk space is being used. Then, click on the option labeled Temporary files.
- Windows will present a list of checkboxes showing various caches you can clear. Look carefully through this list for an option named Previous Windows installation(s). (You will notice this checkbox is taking up an enormous amount of gigabytes).
- Check the box next to Previous Windows installation(s).
- Ensure you uncheck any other boxes (like Downloads) if you do not wish to delete those files.
- Scroll back to the top of the window and click the blue Remove files button.
A warning prompt will appear, verifying that if you delete these files, you will no longer be able to revert your PC to the previous version of Windows. Click Continue. The background process will safely bypass the TrustedInstaller locks, completely obliterate the Windows.old directory, and instantly restore 20+ GB of usable space to your C: drive.