In order to take a screenshot of your desktop or the current window pane on a Mac with Windows installed, the keyboard shortcut is F14. You can find this key just above the keys ‘home’, ‘page up’ etc., above the arrow keys on the apple keyboard.

I found other suggestions on the web for this issue like using Shift--3 or Shift--4 or by using the on-screen keyboard: Programs>>Accessories >> accessibility >> onscreen keyboard
click the ctrl and psc, then normal paste. But, I figured the above mentioned keyboard shortcut is the easiest way to do a print screen.

You might want to go through other third party software like WinSnap or MwSnap to take a screenshot with some useful control options.

Macbook keyboard mapping with Windows