// Variante 1: 
 
procedure TForm1.Button1Click(Sender: TObject); 
var h: HWND; 
begin 
  h := findwindow(nil, nil); 
  repeat 
    if iswindowvisible(h) 
      then Showwindow(h, SW_SHOWMINIMIZED); // nicht SW_MINIMIZE 
    h := getnextwindow(h, GW_HWNDNEXT); 
  until h = 0; 
end; 
 

// Variante 2: Aufwendiger, aber schöner 
 
uses shellapi; 
 
procedure TForm1.Button1Click(Sender: TObject); 
const 
  txt = '[Shell]'#13#10'Command=2'#13#10'[Taskbar]'#13#10 + 
    'Command=ToggleDesktop'; 
  datei = 'weg~tmp.scf'; 
var 
  tf: TFilestream; 
  s: string; 
  buf: array[0..max_path - 1] of char; 
begin 
  zeromemory(@buf, max_path); 
  gettemppath(max_path, buf); 
  s := buf + datei; 
  tf := Tfilestream.create(s, fmcreate); 
  tf.writebuffer(txt[1], length(txt)); 
  tf.free; 
  shellexecute(handle, 'open', pchar(s), nil, nil, sw_hide); 
  deletefile(datei); 
end; 

 
// Variante 3: empfohlen 
 
uses ComObj; 
 
procedure MinimizeAll; 
var 
  Shell: OleVariant; 
begin 
  Shell := CreateOleObject('Shell.Application'); 
  Shell.MinimizeAll; 
end; 
 
procedure TForm1.Button4Click(Sender: TObject); 
begin 
  MinimizeAll; 
end;