To delete the older Windows installation folder you will have to let windows know that its older installation and clean it using disk cleanup.
- Rename the older Windows installation folder to Windows.old
- Now run the disk clean up by right clicking the drive where the old Windows folder exists.
- During disk cleanup select clear system files.
- Now it will detect that there is old Windows folder which can be cleaned.
- Select the Older Windows installation files and complete the disk clean up.
- The Windows.old folder will get deleted automatically.