Here’s the easiest way I can think of to do it. Make a new layer, on top of the “Background” layer with the photo on it. Once you’ve made that layer, make sure it’s selected by left clicking it in the layers palette.

Now, up near the top of the layers window, you should see a place where it says “Normal” and then has an arrow next to it showing that you can change what’s currently selected there via a dropdown menu. Go ahead and do just that – click it. Select “Color”. What you’re doing here is changing the way that this layer interacts with the layers beneath it. Setting it to “Color” makes it so that whatever you put on this new layer will only COLOR the stuff on the layers beneath it, and that’s it.

So, what we’re going to do is select white (or black, or any shade of gray) and paint over the portion of the photo that you want black & white. Doing that will completely desaturate that portion of the photo (since black/white/gray are all completely “colorless” and desaturated), allowing you to keep the colored portion showing through where you want it to and to have the rest (wherever you paint) become black & white.

Ta-da!

Hope that helps.