Print Screen Active Window Only Shortcuts
Print screen the active window only by pressing ALT + Print Screen. On a mac you press Command + Shift + 4 then Space-bar, then click the window.
Just a quick tip for reference. There's a more comprehensive tutorial for macs here: http://osxdaily.com/2010/05/13/print-screen-mac/ or http://graphicssoft.about.com/od/screencapturemac/ht/macscreenshot.htm
If you want to capture a selected area in windows, you can do it if you have One Note installed with Windows + S, or from the Insert menu in OneNote, select screen clipping.
You can also use the screen snipping tool in Windows Vista or 7. Here's a good tutorial on how to assign a shortcut to the snipping tool: