Press print screen. Then press Ctrl+C (copy), open up any image editor (ex: paint, irfanview, etc..), then press Ctrl+V (paste). Then you can just crop the image. (select an area and then crop it)
I recommend irfanview, but MS paint also works.
If you use a mac, you have to use the specific mac commands. They will allow you to control whatever contents you want to capture though, so there should be no problem there.