use ur gun... 
press print screen for the whole screen, alt + print screen for the
active window/dialog box only.
the screenshot is put inside the clipboard, and u can paste it at
mspaint.
if u dunno where the print screen button is, it is above the
ins+del+home+end+pg up+pg dn group of buttons.
for mac, it's command+shift+3 for whole screen, command+shift+4 to
select any part of the screen, command+shift+4+space to select any
open window to capture. screenshots are saved to desktop.