How do I isolate parts of a picture to turn black and white with GNU Image Manipulator?

I want to do the fad that people are doing where people isolate parts of a picture to keep color, then turn the rest black and white (sort of like in the movie Sin City). I have GNU Image Manipulator, but am clueless how to isolate the part i want and turn the rest black and white. If anyone knows how to do this on GNU or on another shareware editor (or anything pre-installed on Windows Vista), can you give me a basic tutorial?