[...]

 > I'm not sure if other OSes support screen capture with the
 > print-screen key.

        For the systems using the X Window System for graphics, xwd(1)
        should do the trick.  (The resulting image should be readable by
        GIMP, http://gimp.org/, ImageMagick, NetPBM, etc.)

        Typically, there is no "system-wide" screen (or window) capture
        key binding for such systems, but certain GUI environments
        provide one anyway.

 > Just did some googling and there is a DOS TSR that can take
 > captures...

 > http://pixelmetrics.com/Snarf/index.htm

        The home page reads:

--cut: http://pixelmetrics.com/Snarf/ --
    Snarf captures full-screen DOS graphics, [...]
--cut: http://pixelmetrics.com/Snarf/ --

        I haven't tried it myself, but I'm not that sure that it is
        capable of capturing /text mode/ screens, which is (AIUI) what
        the OP has requested.

        IIRC, the VGA (EGA) text modes (including mode 3 = 80 x 25) are
        memory-mapped to 0xB8000 (= B800:0000.)  Once the respective
        memory area (which is 80 x 25 x 2 = 4000 bytes long) is dumped
        to a file, one can drop the attribute bytes and use pbmtext(1)
        [1] to convert it to an image.

        Alas, pbmtext(1) is strictly monochrome.  (Perhaps there should
        be ppmtextega(1) and pgmtextmda(1) just as well.)

        For the font, Terminus [2] would be my choice.  (Unless the DOS
        software in question alters the video adapter's font to contain
        any application-specific characters, that is.)

[1] http://netpbm.sourceforge.net/doc/pbmtext.html
[2] http://terminus-font.sourceforge.net/