AFAIK React OS is a different OS. It is nither linux nor windows, but it has binary compatibility with windows 98 and XP as mentioned on their home page. This page will explain more about ReactOS *www.reactos.org/en/about_whatisreactos.html
I tried it sometime back. It can't even detect my USB mouse. So, i had to just throw away it's CD. It has UI similar to windows 98.
Disclaimer from their site
Please bear in mind that ReactOS 0.3.0 RC1 is still in alpha stage and is not recommended for everyday use.