[kalisto]

Código Delphi [-]
function VentanaActiva: String;
var  PC: Array[0..$FFF] of Char;
     Wnd : Thandle;
begin
    {$IFDEF Win32}
    Wnd := GetForegroundWindow;
    {$ELSE}
    Wnd := GetActiveWindow;
    {$ENDIF}
    SendMessage( Wnd , $000D , $FFF , LongInt( @PC ) ); //$000D es el WM_GETTEXT
    Result := PC;
end;