The Cygwin DLL currently works with all recent, commercially released x86 32 bit and 64 bit versions of Windows, with the exception of Windows CE.
What Isn't Cygwin?
What is it?
The Cygwin tools are ports of the popular GNU development tools for Microsoft Windows. They run thanks to the Cygwin library which provides the UNIX system calls and environment these programs expect.
With these tools installed, it is possible to write Win32 console or GUI applications that make use of the standard Microsoft Win32 API and/or the Cygwin API. As a result, it is possible to easily port many significant Unix programs without the need for extensive changes to the source code. This includes configuring and building most of the available GNU software (including the packages included with the Cygwin development tools themselves). Even if the development tools are of little to no use to you, you may have interest in the many standard Unix utilities provided with the package. They can be used both from the bash shell (provided) or from the standard Windows command shell.
What versions of Windows are supported?
Cygwin can be expected to run on all modern 32 bit versions of Windows, except Windows CE. This includes Windows 95/98/ME/NT/2000/XP/2003 and the WOW64 32 bit environment on released 64 bit versions of Windows. As far as we know no one is working on a native 64 bit version of Cygwin. Since Cygwin is a community-supported free software project, patches to provide support for other versions would be thoughtfully considered. Paid support contracts or enhancements are available through Red Hat. For information about getting a Red Hat support contract, see http://cygwin.com/license.html.
Keep in mind that Cygwin can only do as much as the underlying OS supports. Because of this, Cygwin will behave differently, and exhibit different limitations, on the various versions of Windows.