React OS is a free open source operating system based on the best design principles found in the Windows NT architecture (Windows versions such as Windows XP, Windows 7, Windows Server 2012 are built on Windows NT architecture). Written completely from scratch, ReactOS is not a Linux based system, and shares none of the UNIX architecture.
The main goal of the React OS project is to provide an operating system which is binary compatible with Windows. This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, such that people accustomed to the familiar user interface of Windows would find using ReactOS straightforward. The ultimate goal of React OS is to allow you to use it as alternative to Windows without the need to change software you are used to.
goto reactos.org for more information