[WebMethod]
        public string HelloWorld()
        {
            System.Threading.Thread.Sleep(10000);
            return "Hello World";
        }

private void button1_Click(object sender, EventArgs e)
        {
            for (int i = 0; i <= 500; i++)
            {
                BackgroundWorker bk = new BackgroundWorker();
                bk.DoWork += new DoWorkEventHandler(bk_DoWork);
                bk.RunWorkerAsync();
            }          
        }

        void bk_DoWork(object sender, DoWorkEventArgs e)
        {           
            ServiceReference1.Service1SoapClient cl = new WindowsFormsApplication1.ServiceReference1.Service1SoapClient();            
            BeginInvoke(new UpdateTextDelegate(UpdateText), new object[] { cl.HelloWorld() });
            
        }