You can use 'Ubuntu Mini Remix'. It has only Command Line Interface(CLI). You can use it with virtual box or with live CD or pendrive.
After booting the OS you have to put the username 'ubuntu' and password should be blank, simply hit the enter key.
After the login it looks like this.
It uses so little memory (mine is 21mb) that you can use it on any old system.
To check the ram uses, use this command:
You can shutdown the system with the following command:
sudo shutdown -h now
To restart the OS use this command:
sudo shutdown -r now
You can download 'Ubuntu Mini Remix' from here
This is a live operating system, so it will not ask you to install it in your hard disk. But you can install it manually with this guide.
But if you want to installed the system in your hard disk, I will recommend you to install 'ubuntu mini' without any desktop environment like Gnome,XFCE,Mate,KDE etc (It will ask you to choose the desktop environment while installing the OS). You can download Ubuntu Mini from here.
Remember, you need an active internet connection while installing 'Ubuntu Mini'. So you can not install Ubuntu Mini offline.
It is best to install 'Ubuntu Server' if you want to install the system offline and you want to install the OS inside of your hard disk. You can install Ubuntu Server from this link.