ReactOS is an Open Source Operating System that aims for compatibility with Windows NT 5.2 (that is Windows XP/2003). It does have some newer features as well. At this time it is still labeled as being in the Alpha Stage of development and testing. It is not recommended for use as a primary OS.
Despite its still in development nature, I use ReactOS as one of my main Operating Systems. I am running it on an Intel Atom based Laptop computer. In order to get it to run I had to modify it enough that it is not very useful as a test target, as my config is significantly different from the standard ReactOS install.
Because of the differences in the version I boot natively and normal ReactOS, I also run a normal version of ReactOS in emulation on ReactOS, so I have a testworthy target. I am not sure which is better, though they both work and are usable for my purposes.
In most cases I would say for the time being stick with your prefered OS. If you are a user of MS-Windows, then it is worth keeping an eye on ReactOS. It is possible that one day you may be interested in trying ReactOS as an alternitive to Windows for a number of potential reasons. Some people just want something completely Open Source though can not leave Windows behind for whatever reason. Others will like the licensing for there use, perhaps saving them money in there installation.
If you wish to try out ReactOS it is likely best to run it in VirtualBox for the time being. ReactOS is fairly well tested in VirtualBox, and there is even a guide for setting ReactOS up in VirtalBox on the ReactOS site, see: VirtualBox - ReactOS Wiki.
The biggest advantage of ReactOS is that it is Open Source, while being able to use most drivers written for MS-Windows, and being able to run more and more of the software written for Windows. As we are all aware, on Intel and AMD based systems most drivers are written for MS-Windows, so this advantage can not be understated. There are also a lot of comercial and open source games that only run on Win32 compatible OSes, as well as some other software for which this is true.
Despite being highly compatible with Windows NT, ReactOS is a different codebase, created from the ground up. This makes it so that many of the well known security holes in MS-Windows simply do not exist in ReactOS. ReactOS does have its own troubles of course, though they are often different from those of Windows.
ReactOS has much lower system requirements than MS-Windows XP or 2003. ReactOS will run well on a system having only 128MB of ram and a 200MHz PII CPU. Though ReactOS deos need at least a VESA 2.0 complient video card, so this could be an issue with some older systems. A standard install of ReactOS only takes up about 400MB of space including drivers needed, adding a web browser only adds 90MB for 490MB total. As such it runs extremely well on "Modern" systems, though does not support UEFI boot.
For those that remember the old days;
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