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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.FormCreate(Sender: TObject);
begin
  ShowWindow( Application.Handle, SW_HIDE );
  SetWindowLong( Application.Handle, GWL_EXSTYLE,
                 GetWindowLong(Application.Handle, GWL_EXSTYLE) or
                  WS_EX_TOOLWINDOW and not WS_EX_APPWINDOW);
   ShowWindow( Application.Handle, SW_SHOW );
end;                    

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procedure TForm1.FormShow(Sender: TObject);
var Owner : HWnd;
begin
  Owner:=GetWindow(Handle, GW_OWNER);
  ShowWindow(Owner, SW_HIDE);
end;