http://www.gimp.org/windows/

GIMP for Windows free graphics design & photo editor
GIMP is a popular open-source image editor originally developed for Unix/Linux. Often lauded as the "free Photoshop," it does have an interface and features similar to Photoshop. Because it's volunteer-developed beta software, stability and frequency of updates could be an issue; however, many happy users report using GIMP for Windows without significant problems. The GIMP's user interface is designed to let you work as efficiently as possible. The trade-off is that first time users need an introduction to the GIMP's user interface, such as the one written by M. Weisbeek.
The GIMP should run on Windows 98/ME/NT4/2000/XP. It is recommended to use an NT-based version of Windows (NT4, 2000 or XP). Starting with GIMP 2.2 and GTK+ 2.4, older versions such as Windows 95 are not supported anymore. The amount of memory can be an important factor, especially if you intend to work on large images. A minimum of 128 MB of RAM is recommended, but the more RAM the better. 
The first time you run the GIMP. When you run the GIMP for the first time, it will install some files in your home directory and it will show you some dialogs that give you the opportunity to adapt it to your environment. 
How to set the tile cache. The tile cache tells the GIMP how much memory it can use before swapping some image data to disk. Setting the size of the tile cache can be critical for the performance of your system. 

This site, www.gimp.org, only distributes the source code to the GIMP (the building blocks). You can however download executable versions

GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages.