1.

 How to fix Error 0x80070091 The directory is not empty. - The easy way!

2.

Open an elevated command prompt as admin.

Copy and paste the commands below into the elevated command prompt one at a time, and press Enter after each command.

If you had another say folder that you also wanted to delete, then you could repeat these commands substituting that name instead of windows.old to also delete it.

takeown /F "C:\Windows.old" /A /R /D Y

icacls "C:\Windows.old" /grant *S-1-5-32-544:F /T /C /Q

RD /S /Q "C:\Windows.old"

AND

takeown /F "C:\$Windows.~BT\*" /A /R /D Y

icacls "C:\$Windows.~BT\*.*" /grant *S-1-5-32-544:F /T /C /Q

RD /S /Q "C:\$Windows.~BT"

When finished, you can close the elevated command prompt if you like.

 3.

Download winold.bat & run as Admin