If your recently installed OS is working fine as expected, it is better to delete your Windows.old folder permanently which is eating your 9 to 25 GB that is quite a large space.
Warning: Windows.old folder can’t be recovered once deleted!
How to Delete the Windows.old system folder
1. Click on start button and search for disk cleanup hit enter to open the disk cleanup drive selection.
2. Select the drive where your previous Windows was installed and click on OK. Then system will searching for system error memory dump files.
3. Then disk cleanup wizard will appear. Click on “Clean up System files“.
4. Again select the drive if you are prompted and click on OK. That will scan your previous Windows files.
5. Now select the option Previous Windows installation(s) and click on OK.
6. When the message appears for confirmation, click on Delete files.
Now Windows.old folder will be deleted forever which will give you quite a large space in your system.