If you are adventurous you can try using an elevated Command prompt.

Step 1: Open elevated Command Prompt. To do so, type CMD in taskbar search box or Start search box and then simultaneously press Ctrl+Shift+Enter keys to open Command Prompt as administrator. Click Yes button when you see the UAC dialog.

Step 2: In the Command Prompt, paste the following command and then press Enter key to delete Windows.old folder.

RD /S /Q %SystemDrive%\windows.old

Or to take ownership of a folder if that won't work.

In the elevated command prompt, type the command you want below, press Enter.
NOTE: Substitute full path of folder or drive with the full path of the folder or drive letter that you want to take ownership of within quotes.


(To grant currently logged on user ownership of)

takeown /F "full path of folder or drive" /R /D Y


(To grant administrators group ownership of)

takeown /F "full path of folder or drive" /A /R /D Y