Can I install another OS on Android? I have a Samsung galaxy note 3 which has root access (I think, I mean I can access and change files in /root by the help of an installed SuperSU) and I'm wondering if I can install and .ISO from a DVD, I do have a USB DVD drive and the required adapter, I'm planning on installing Ubuntu Desktop or a version of Windows.

Thanks in advance.
Yes, you can use a custom ROM on your Android device.

You also need a custom kernel, that supports multiboot/multi-OS and you need a special Ubuntu build with ARM and hardware support. It may be hard to find but I don't think it's impossible. reference site: Asus Zenfone Blog