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function TForm1.SucheFensterUeberTitel(FensterTitel: string): Hwnd; 
var 
  NextHandle: Hwnd; 
  NextTitle: array[0..260] of char; 
begin 
  NextHandle := GetWindow(Application.Handle, GW_HWNDFIRST); 
  while NextHandle > 0 do begin 
    GetWindowText(NextHandle, NextTitle, 255); 
    if Pos(FensterTitel, StrPas(NextTitle)) <> 0 then begin 
      Result := NextHandle; 
      Exit; 
    end else 
      NextHandle := GetWindow(NextHandle, GW_HWNDNEXT); 
  end; 
  Result := 0; 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  if SucheFensterUeberTitel('Arbeitsplatz') <> 0 then  
  Showmessage('Arbeitsplatz offen'); 
end;                    

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procedure TForm1.Button1Click(Sender: TObject); 
var h: hwnd;
begin 
  h := SucheFensterUeberTitel('Microsoft Excel');
  if h <> 0 then postmessage(h,WM_CLOSE,0,0); 
end;