INT 2F - MS Windows v3.0 - INSTALLATION CHECK
        AX = 4680h
Return: AX = 0000h MS Windows 3.0 running in real (/R) or standard (/S) mode,
                   or DOS 5 DOSSHELL active
           nonzero  no Windows, Windows prior to 3.0, or Windows3 in enhanced
                    mode
Note:   not officially documented, but Microsoft has indicated that they
          intend to provide an installation check which works in all modes