The Gimp is GNU's answer to Photoshop.
GNU (which stands for GNU's not Unix) or the free software foundation was started by anarchistic programmers that believed that to own intellectual material was (is) wrong (whatever your opinion on this, a lot of free (software that doesn't cost anything as well as is free to develop) is handy). Open source is a more level headed intuitive than the GNU project as it was designed to encourage businesses to release the source code. But The Gimp is a great program

Now I'm not sure about a Windows version but linux isn't hard. Check the Latest distros, they beat Win XP for looks as well as performance and reliability
