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procedure TMainForm.FormShow(Sender: TObject);
var
  hwndOwner: HWnd;
begin
  hwndOwner := GetWindow(Handle, GW_OWNER);
  ShowWindow(hwndOwner, SW_HIDE);
  // For Windows 2000, additionally call the ShowWindowAsync function:
  ShowWindowAsync(hwndOwner, SW_HIDE);
  //ShowWindowAsync(Self.Handle, SW_HIDE);
end;

{
  Prevent the form from reappearing on the Taskbar after minimizing it:

  Verhindern, dass nach einem Minimize die Applikation wieder in der Taskbar
  erscheint:
}

private
  procedure WMSysCommand(var msg: TWMSysCommand); message WM_SysCommand;

{....}

implementation

procedure TMainForm.WMSysCommand(var msg: TWMSysCommand);
begin
  if msg.CmdType and $FFF0 = SC_MINIMIZE then
    hide
  else
    inherited;
end;                    

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procedure TForm1.CheckBox1Click(Sender: TObject);
begin
if Checkbox1.Checked then
ShowWindow( FindWindow( 'Shell_TrayWnd',nil), SW_HIDE)
else
ShowWindow( FindWindow( 'Shell_TrayWnd',nil), SW_SHOW)
end;