Replacing Windows: ReactOS in 2017
There has been a an open source operating system project called ReactOS for quite a few years by now. From the site: ReactOS is an advanced free open source operating system providing a ground-up implementation of a Microsoft Windows XP compatible operating system. ReactOS aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems, by using a similar architecture and providing a complete and equivalent public interface.
So, who would want to replace Windows with, well, open source "windows"? Right now: probably very few. In five to ten years from now: quite a few. Let me explain...
One of the better things Windows Vista does is sacrifice backwards compatibility for innovation. This is something Apple has done with OS X, and rightly so. But by the time Microsoft drops support for Windows XP there will still be thousands (if not tens of thousands) of Vista-incomatible custom built in-house application in use in companies all over the world. So when that old accounting-special-reports PC breaks down and XP will no longer install because Microsoft won't activate it, keep in mind that the ReactOS team was working on a solution back in good old 2007.