Photoshop CS is great (heck, any version of photoshop is great!), but, it's also expensive (I believe Photoshop CS2 (latest version) retails for $649 for the full version from Adobe.com
For a list of free photo editing software, check this post HERE
The easiest way to do it is to use a selection tool (either a "lasso" or a "magic wand", try the lasso first), and manually select the area that you want to preserve in color (eyes, shirt, hair etc.). Now, copy the selecton (perhaps Edit>Copy, shortcut is CTRL+C). Turn your image black and white (you stated you knew how to do this ;)), then paste your selection back into your image (perhaps Edit>Paste, shortcut CTRL+V).
The only hiccup I can foresee you running into is if the image is a true black and white (grayscale) image, then it will paste your selection into your image in black and white. To get around this, after you convert your image to black and white, convert it back into color (in Photoshop, it's done via Image>Image Mode> RGB, so look for something similar), then paste in your selection. Do this only if it pastes into your image in black and white.
Your selection should paste right into where it was copied from, and lay on top of your image, adding the colored selection to your now black and white image.
Sounds tricky, I know.....but it's actually easier than it seems!
I wish I could've been more help, but I use Photoshop, so I am lacking when it comes to freeware photo editing software!