is there a such thing as Open Source Windows OS? yes there is, it is called ReactOS. ReactOS is an open source operating system designed on the architecture of Windows NT, that means that all windows based applications will be compatible with the OS. unlike Linux where nothing from windows is compatible, ReactOS has great potential to be popular among the public. also being built on the great NT architecture it promising good security features.
according the their site, the ultimate goal of ReactOS is to replace the windows operating system on a computer without a user even noticing. check some screenshots of the Alpha stage:
you can check a lot more screenshots from here http://www.reactos.org/en/screenshots.html
you can download the Alpha package from here http://www.reactos.org/en/download.html
remember, the software is still in Alpha stage, which means it is not suitable for production use, and don’t get frustrated if it does not work on things you try to do. I hope the project gets to maturity, so we can have an alternative to Windows.