远程服务器返回错误:501

   System.Net.ServicePointManager.Expect100Continue = false;
                WebClient webClient = new WebClient();
                 webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
                webClient.Headers.Add("Content-Type", "charset=UTF-8");
                webClient.Headers.Add(HttpRequestHeader.KeepAlive, "false");
                webClient.Headers.Add("Cache-Control", "no-cache");
                webClient.Headers.Add("User-Agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
                byte[] postData = Encoding.UTF8.GetBytes(postString);
                byte[] responseData = webClient.UploadData("http://xxx.com/a.aspx", "POST", postData);
                string srcString = Encoding.Default.GetString(responseData);
用webclient访问网页时，在一些电脑上会出现错误：远程服务器返回错误:501(未实现)，而一些电脑上却完全正常，求解决办法，谢谢！