http post接口的实现问题？

protected void btnLoad_Click(object sender, EventArgs e)
 {
        //入口地址为：http://125.208.3.12:8888/sms.aspx
        string url="http://125.208.3.12:8888/sms.aspx?action=overage&userid=1152&account=111&password=111";
        Response.ClearContent();
        Response.Write(Post(urll));
        Response.End();
 }
   

public string Post(string url)
    {
        string returnData = null;
        try
        {
            HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(url);
            webReq.Method = "POST";
            webReq.ContentType = "application/x-www-form-urlencoded";
            Stream postData = webReq.GetRequestStream();
            postData.Close();
            HttpWebResponse webResp = (HttpWebResponse)webReq.GetResponse();
            Stream answer = webResp.GetResponseStream();
            StreamReader answerData = new StreamReader(answer);
            returnData = answerData.ReadToEnd();

        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }
        return returnData.Trim()+"\n";
    }
现在的问题是它return是xml形式的string，我怎样得到它标签里边的值？还有如果post(string url)的url改为入口地址,那参数怎么办,这方法又该怎么写？小生不才,望同业好手指点迷经，小弟在线等..