ReactOS is an interesting open source project which offers a completely free operating system on your PC based on Windows NT format. It is completely compatible with drivers and controllers so you won't have any problems getting your internal and external components to function that previously worked with NT.
The operating system has the traditional look of Windows and is based around the concept of a desktop, toolbar, start menu etc. You can install and run all the applications that are supported in NT and the developers plan to support Windows 2000 and XP in the future. They stress however that at the moment, ReactOS is still in alpha stage, meaning it is not feature complete and is not recommended for everyday use.
This is one of the problems you will find on using it as it still prone to bugs and crashes especially when installing software. Nevertheless, this is an exciting project and gives hope to all those who are tired of using Windows.