Back in the days of DOS, the PrtScn key used to print the screen. But
in all versions of Windows, this works differently, and the name of
the key is now an anachronism. The key does *not* print the screen.
PrtScrn captures the whole screen, and Alt-PrtScrn captures the active
window.
Either one captures the image to the Windows clipboard. Once it's in
the clipboard you can paste (Ctrl-V) it into any application that
supports graphics (Windows Paint, other graphics programs, even your
favorite word processor). You can edit or add to the image as you
wish, then print it, or in your case, save it and attach it to an
e-mail message.
This ability to manipulate the image in a program before printing it
is an improvement over the original DOS method of just printing it.
But if you'd like that old facility back, there are several
third-party freeware/shareware programs that can do this, such as
PrintKey2000 at
http://www.sharewarejunkies.com/00zwd2/printkey2000.htm