type
   TForm1 = class(TForm)
   public
     procedure WMSize(var M : TWMSIZE) ; message WM_Size;
   end;

implementation

procedure TForm1.WMSize(var M : TWMSIZE) ;
begin
  if M.SizeType=Size_Minimized then
  begin
    ShowWindow(Handle,Sw_Hide) ;
    M.Result := 0;
  end
  else
    inherited;
end;