Pressing the Alt+PrintScreen Keys together should copy the active window into the clipboard, this can then be pasted into your image editor (Paint, not familiar with this if it can do it).

This is the freeware screen capture program that I use ScreenPrint (Technical recommended it), it is very handy, easy to setup and use, when you click the Prt Sc key it will save it in the file type (.jpg, .gif, etc.) you chose when you setup the program.