If you dont want to buy a new usb or wireless keyboard and use it with your macbook just to take screen shots of your desktop by print screen, simply follow the following:
When on apple macbook and logged into windows (xp, vista, seven) simply press fn + shift + f11 keys. This is an alternate to print screen on macbook and other systems / laptops. Then you can paste the copied image to your favourite image editing prgram like mspaint etc.