嗯，仅仅覆盖Form.ShowWithoutActivation还不够吗？

protected override bool ShowWithoutActivation
{
  get { return true; }
}

而且，如果您也不希望用户单击此通知窗口，则可以覆盖CreateParams：

protected override CreateParams CreateParams
{
  get
  {
    CreateParams baseParams = base.CreateParams;

    const int WS_EX_NOACTIVATE = 0x08000000;
    const int WS_EX_TOOLWINDOW = 0x00000080;
    baseParams.ExStyle |= ( int )( WS_EX_NOACTIVATE | WS_EX_TOOLWINDOW );

    return baseParams;
  }
}