Also see: WINE | Windows | Emulation

Home Page:

Note: Currently in the alpha development stage.

From the homepage:

ReactOS is an open-source effort to develop a quality operating system that is compatible with Windows NT applications and drivers.
Relationship with the WINE project: ReactOS has always planned to work with the WINE project to share as much programming effort as possible. This will mainly concern User Mode DLLs and will happen once ReactOS’s Kernel Mode areas are more complete, as those areas form the underlying infrastructure. Other areas of cooperation lie in applications and testing suites.
Future compatibility: The ReactOS project, although currently focused on Windows NT 4.0 compatibility, is always keeping an eye towards compatibility with future Windows NT releases, that is, Windows 2000 (NT 5.0) and Windows XP (NT 5.1).
Supporting other System Applications: The Windows NT architecture allows for subsystems, as does the ReactOS architecture. A subsystem is an implementation of the APIs of another operating system, allowing ReactOS to run applications from other systems. We are already looking at subsystems for: Java, OS/2 and DOS and possibly others in the future.”

Hardware Requirements

Whether installing ReactOS on real hardware or on an emulator, it is important to know the minimum requirements to install and use ReactOS:

TakeDown.NET -> “ReactOS