clipped from www.gnu.org

Microsoft Windows is a clear and instructive example of non-free software. Its source code is a secret, so programmers cannot learn from it, fix it, adapt it to their clients (your) needs, or even verify what it really does. If you share copies with your neighbors, you will be called a "pirate", and users have been threatened with imprisonment for this. Non-free software is completely controlled by its developer, who also has power over the users. We started the free software movement because this power is unjust.

Using free software on Microsoft Windows (or any non-free operating system) is the first step towards freedom, but it does not get you all the way there. You're still under Microsoft's power as long as you use Windows. So the next step is to replace Windows with a free operating system such as GNU/Linux. However, on this page we're concerned with the first step.