Can't retrieve Full User Name from Active Directory

For the past few days I've been trying to use Asp.net to get the full user name of a user logged onto an intranet website.  I've disabled anonomous access and enabled integrated windows authentication.  I'm able to get the logon name with Request.ServerVariables["LOGON_USER"], and have added <identity impersonate="true" /> to web.config.  After trying many different examples I finally got the following code to return the Full User name in Visual Studios debug mode, but when I try to use the same code from IIS the GetFullName function returns "System.Runtime.InteropServices.COMException (0x80070035):"  Does anyone have any idea how to get this running?


private void Page_Load(object sender, System.EventArgs e)
            {
                  
                  Label1.Text = GetFullName(Request.ServerVariables["LOGON_USER"].ToString());

            }

            private string GetFullName(string strLogin)
            {
                  string str = "";
                  // Parse the string to check if domain name is present.
                  int idx = strLogin.IndexOf('\\');
                  if (idx == -1)
                  {
                        idx = strLogin.IndexOf('@');
                  }

                  string strDomain;
                  string strName;

                  if (idx != -1)
                  {
                        strDomain = strLogin.Substring(0, idx);
                        strName = strLogin.Substring(idx+1);
                  }
                  else
                  {
                        strDomain = Environment.MachineName;
                        strName = strLogin;
                  }

                  DirectoryEntry obDirEntry = null;
                  try
                  {
                        obDirEntry = new DirectoryEntry("WinNT://" + strDomain + "/" + strName);
                        System.DirectoryServices.PropertyCollection  coll = obDirEntry.Properties;
                        object obVal = coll["FullName"].Value;
                        str = obVal.ToString();
                                                                return str;
                  }
                  catch (Exception ex)
                  {
                        str = "";
                        return "Error: "+ex;
                  }
                  
            }