procedure TForm1.Button1Click(Sender: TObject);
var
  h: HWnd;
begin
  h := Handle;
  while h > 0 do
  begin
    if IsWindowVisible(h) then
      PostMessage(h, WM_SYSCOMMAND, SC_MINIMIZE, 0);
    h := GetNextWindow(h, GW_HWNDNEXT);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Keybd_event(VK_LWIN, 0, 0, 0);
  Keybd_event(Byte('M'), 0, 0, 0);
  Keybd_event(Byte('M'), 0, KEYEVENTF_KEYUP, 0);
  Keybd_event(VK_LWIN, 0, KEYEVENTF_KEYUP, 0);
end;