function EnumMiniProc(Wnd : HWND; Param : Longint) : Boolean; stdcall;
begin
  Result := True;
  if (Wnd = Param) or (Wnd = Application.Handle) then
    Exit
  else if isWindowVisible(Wnd) and not isIconic(Wnd) and isWindow(Wnd) then
    ShowWindow(Wnd, SW_MINIMIZE);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  EnumWindows(@EnumMiniProc, Handle);
end;