The React OS project is an operating system written from scratch that follows the design and architecture of Windows NT. The idea behind it was to launch a free and open-source operating system that would enable windows programs and drivers to run on your PC.
Now, what you must be thinking is: “Why the hell would one want to go for such an OS when we already have got Windows?” Well, that’s cause ReactOS is free and open-source, means that you are free to customize it to suit your needs. It’s not just an alternative, but a free alternative to Microsoft Windows.
It’s pretty light actually. You can download it from the reactOS site, though i’d advise you to try it on a VM first, cause you might not find it as interesting as it sounds.