From LQWiki
ReactOS is an open source attempt to develop an operating system that is compatible with Windows NT applications and drivers, specifically NT 4.0. It is licensed under the GPL. The ReactOS project shares a lot of code with the WINE project.
The main reason NT 4.0 is the target is that it is the bare minimum. If a Windows program works with NT 4.0 it will work with almost every Windows version. Once ReactOS can run NT 4.0 programs, they can start to add additional functionality.
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