There have been Windows installers for quite a while: http://gimp-win.sourceforge.net/stable.html You'll see the 2.6 stable is available at the top and 2.8 RC1 is at the bottom (I'm guessing 2.8.0 will be up by week's end.)
There are a lot more alternatives on Windows though, Paint.net and Photoshop being obvious choices. With Linux and similar Gimp is really the only image editor. That I know of anyway.
