C#/.NET WinForms Application to login to a forum using HttpWebRequest and POST

Hi,

I am working on building a C# Winforms application which will allow me to login to a forum or site and perform certain tasks. I am stuck at the very 1st step. I cannot login to the forum. Here is what I have tried so far,
the Login button has the following code :-

            try
           {
               Uri uri = new Uri("http://www.thinkdigit.com/forum/login.php");
               string data = "vb_login_username=userid&vb_login_password=password";
               HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(uri);
               request.Method = WebRequestMethods.Http.Post;
               request.ContentLength = data.Length;
               request.ContentType = "application/x-www-form-urlencoded";
               StreamWriter writer = new StreamWriter(request.GetRequestStream());
               writer.Write(data);
               writer.Close();
               HttpWebResponse response = (HttpWebResponse)request.GetResponse();
               StreamReader reader = new StreamReader(response.GetResponseStream());
               string tmp = reader.ReadToEnd();
               response.Close();
               richTextBox1.AppendText(tmp);
           }
           catch (Exception ex)
           {
               textBox3.Text = ex.ToString();
           }

Thats what I scrounged up after google busting and visiting so many forum posts. Unfortunately this does not seem to log me in the forum. I am checking the HTML generated content in t my richtextbox control and there's the login fields there still. Which means it didn't login.

I checked the login page.

and these are the contents there