Excerpting Wikipedia, ReactOS is: an open-source operating system intended to be binary-compatible with computer programs and device drivers made for Windows. ReactOS is primarily written in C. The project has been ported to the ARM and AMD64 processor architectures, and partially implements Windows API functionality. The latter is assisted by including parts from the Wine compatibility layer. The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows … such that people accustomed to the familiar user interface of Windows would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to remove Windows and install ReactOS without the end user noticing the change.