Не получается отправить Post запрос - C#

POST /choice/check HTTP/1.1
Host: www.bochkavpechatleniy.com
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:11.0) Gecko/20100101 Firefox/11.0
Accept: */*
Accept-Language: en-us,en;q=0.7,ru;q=0.3
Accept-Encoding: gzip, deflate
DNT: 1
Connection: keep-alive
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Referer: http://www.bochkavpechatleniy.com/choice
Content-Length: 107
Cookie: uniq_id=11075584; __utma=221629963.2098014455.1332771477.1332819560.1332822127.4; __utmz=221629963.1332771477.1.1.utmcsr=e.mail.ru|utmccn=(referral)|utmcmd=referral|utmcct=/cgi-bin/msglist; _user_birthday_date=02; _user_birthday_month=02; _user_birthday_year=1967; _user_country=150; _user_language=45; PHPSESSID=lu9grejkptpq46o39tbh2noip6; __utmc=221629963; __utmb=221629963.2.10.1332822127
Pragma: no-cache
Cache-Control: no-cache

user_country=150&user_language=45&user_birthday_date=4&user_birthday_month=2&user_birthday_year=1971&ajax=1

  private void button1_Click(object sender, EventArgs e)
        {
            WebRequest webreq = WebRequest.Create("http://www.bochkavpechatleniy.com/choice");
            webreq.Method = "POST";
            webreq.Timeout = 120000;
            webreq.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
            byte[] sentData = Encoding.UTF8.GetBytes("user_country=150&user_language=45&user_birthday_date=4&user_birthday_month=2&user_birthday_year=1971&ajax=1");
            webreq.ContentLength = sentData.Length;
 
            Stream sendstream = webreq.GetRequestStream();
            sendstream.Write(sentData, 0, sentData.Length);
            sendstream.Close();
            WebResponse webresp = webreq.GetResponse();
            //webBrowser1.Navigate(webresp.ResponseUri);
            Stream respstream = webresp.GetResponseStream();
            //webBrowser1.DocumentStream = respstream;
            StreamReader str = new StreamReader(respstream);
            richTextBox1.Text = str.ReadToEnd();
            str.Close();
            respstream.Close();
        }