401 rest service: works in browser, error from code

I am trying to consume a REST service from another company. When I use the link in the browser it works fine. However I get the 401 error when accessing from code. What am I doing wrong? Do I need to add something to the web.config of the web app?

            string URL = @"https://hdapps-qa.homedepot.com/MYTHDPassport/rs/identity/isSessionValid?thdsso=TEST&callingProgram=PSC&Submit=Submit";

            HttpWebRequest req = WebRequest.Create(URL)

                                 as HttpWebRequest;

            string result = null;

            req.Method = "GET";

            req.UseDefaultCredentials = true;

            req.PreAuthenticate = false;

            req.Credentials = CredentialCache.DefaultCredentials;

            using (HttpWebResponse resp = req.GetResponse()

                                          as HttpWebResponse)

            {

                StreamReader reader =

                    new StreamReader(resp.GetResponseStream());

                result = reader.ReadToEnd();

            }