string url=http://www.csdn.net/fly.aspx?id=515225;

GetPageHtml(url);

private String GetPageHtml(string url)
  {
   String Result;
   WebResponse MyResponse;
   WebRequest MyRequest = System.Net.HttpWebRequest.Create(url);
   MyResponse = MyRequest.GetResponse();

using (StreamReader MyReader = new StreamReader(MyResponse.GetResponseStream(),System.Text.Encoding.Default))
   {
    Result = MyReader.ReadToEnd();
    MyReader.Close();
   }
  
   FileStream fs = new FileStream("c:\\flystudio.htm",FileMode.Create,FileAccess.Write);
   StreamWriter sw = new StreamWriter(fs,System.Text.Encoding.Default);
   sw.WriteLine(Result);
   sw.Close();
   fs.Close();
   return Result;
  }

解释请查看vs.net自带的msdn......