相关疑难解决方法(0)

我正在尝试获取某些网页的html代码,我有一个正确的用户名和密码,但我仍然无法让它工作,这是我的代码:

private void buttondownloadfile_Click(object sender, EventArgs e)
{
    NetworkCredentials nc = new NetworkCredentials("?", "?", "http://cdrs.globalpopsvoip.com/0000069/20091229/20091228_20091228.CDR");   
    WebClient client = new WebClient();

    client.Credentials = nc;
    String htmlCode = client.DownloadString("http://cdrs.globalpopsvoip.com/0000069/20091229/20091228_20091228.CDR");

    MessageBox.Show(htmlCode);
}

MessageBox只是为了测试它,问题是每次我到达这一行:

String htmlCode = client.DownloadString("http://cdrs.globalpopsvoip.com/0000069/20091229/20091228_20091228.CDR");

我得到一个例外:

远程服务器返回错误:(401)未经授权.

我该如何解决？