Some notes on Linux - Opensource and the whole GNU :)
There are also light weight "windows managers" which work as light weight desktops. Enlightenment, JWM, Fluxbox, Iceworm. I prefer XFCE because it is highly configurable for the resources used. It is fairly intuitive as well. Now for distro choices: If you have a Broadcom card look to Mint 17.3 or 18.1 in XFCE, Peppermint 7, LXLE an LXDE dt, Korora in XFCE. These three work well out of box and a good place for starters. With more experience, antiX and Bodhi are super light weight in ram usage, Manjaro in XFCE, Mageia in XFCE, Mate, Cinamin, Gecko in XFCE. These distros offer great features but needed some extra work, such as loading Codecs for audio, or that they lack in Broadcom wifi card support, or I needed to install some support software such as spell check software. These distros are worth the extra effort if you have enough experience, or are adventurous. As of this writing I am testing antiX 16. It looks to be the king of lightweight. It starts at well under 100 megs. I like the Rox Fluxbox desktop.Tool to make it easy to add feature and programs.