There are a few old threads on this topic, you might be able to find them with a search.
It's actually pretty easy with software like Photoshop. You can make a duplicate layer of the image and desaturate the new layer. (there are a few different ways to convert to b&w so read up on that as well) Now all you have to do is to erase,from the b&w layer, the parts that you want to be in color.
An even better way would be to use a layer mask. It's almost the same as erasing from the b&w layer except that you can add or subtract from the mask as you go.
There are plenty of methods to select the area you are erasing or masking. Magic wand, brush tool, lasso.