As with many things in photoshop, there are a number of ways to do it.
I am assuming that you are starting with a color photograph
For the purpose of explaining this method I am using this photo for no other reason than the idea that it has an item that will stand out better with a B n W background. I am using photoshop 7.0.
Open the image you are going to work with.
Create a new layer to work with. You can creat a duplicate layer and work with it exactly like I describe. I chose to use an adjustment layer. (red arrow)
You are then going to turn this new layer into a black and white layer useing the method you prefer..
layer>new adjustment layer>hue/saturation
This opens a dialogue box where I can turn the saturation all the way down. (blue arrow)
Remeber, you color layer still exists below this new black and white layer.
Now with this new layer selected, use the eraser tool (blue arrow). adjust the brush size to something you can work with. Adjustments to the flow and opacity of the brush will adjust the degree of the effect. I have them both set at 100 just for the sake of illustration.
Go over the portion of the photo that you want in color with the eraser. This allows the color layer below to show through.
When you have erased all of the items in the photo you would like to have in color, go to layers and select flatten image. This will merge all of the layers into one layer and allow you to save the file. Choose save as or you will overwrite the original file and you will no longer have the color version.
I did this pretty quick and would have done a better job of erasing if this had been my own project, but it works for the purpose of explaining.
You should end up with something like this.
I hope that helps at least get you started