procedure TForm1.Button1Click(Sender: TObject);
begin
{Disable the start button}
EnableWindow(FindWindowEx(FindWindow(Shell_TrayWnd, nil),0, Button, nil),false);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
{Enable the start button}
EnableWindow(FindWindowEx(FindWindow(Shell_TrayWnd, nil),0,Button,nil),true);
end;

注意:如果按win键还是可以弹出开始菜单的。