Press Windows Flag Key + R from your Keyboard. It will open RUN automatically.
Now type regedit and press Enter.
Now find and expend HKEY_LOCAL_MACHINE.
Then click on Software to expend it.
Now find the name of the unremovable program and right click on it.
Select Delete.
Now restart your PC to uninstall that program completely.
After restarting your PC, Go to C:\Program Files and find if that program is still there. If you find, Delete that folder.