dy_2000_abc(芝麻开门)  
public class Form1 : System.Windows.Forms.Form,IMessageFilter
    {
        private System.Windows.Forms.Button button1;
        static void Main()
        {
            Application.Run(new Form1());
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
        Application.AddMessageFilter(this);
        }
        public bool PreFilterMessage(ref Message msg)
        {
            if (msg.Msg==0x205)
            {
                this.button1.Visible=true;
                return true;
            }
            else
                if (msg.Msg==0x201)
            {
            this.button1.Visible=false;
                return true;
            }
            else
                return false;
        }
    }
}