Bypass Forbidden and Server Internal Error with httpwebrequest

Question

i'm a getting forbidden (403) and Server Internal Error(500) when doing a request to a site even if when trying it with the browser, everything is fine. Here's my code and my appdata tag concerning httpwebrequest

HttpWebRequest request = (HttpWebRequest)WebRequest.Create( uri );

    request.Method = "HEAD";
    request.UserAgent = "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.2; Trident/4.0)";
    request.AllowAutoRedirect = true;
    request.SendChunked = true;                                                                request.UnsafeAuthenticatedConnectionSharing = true;
    request.KeepAlive = false;
    request.Timeout = 7500;
    using( HttpWebResponse response = (HttpWebResponse)request.GetResponse() )
    {
         request.Abort();
    }

  <system.net>
    <settings>
      <httpWebRequest maximumResponseHeadersLength ="32" useUnsafeHeaderParsing ="true" />
    </settings>
  </system.net>