Version: (using KDE 4.2.0)
OS: MS Windows
Installed from: MS Windows
Porting kwin to Windows is a required step to allow users to replace the Windows shell with KDE.
Let's face it: this will never happen.
KWin source code has been written with only X11 in mind. So porting to Windows would be more difficult than writing a new window manager for Windows.
Adding support for Windows platform would make KWin an unmaintainable monster and that would neither help Linux nor Windows users.