Hello, Shashank,
Generally error message 401 means you need to log on (enter a valid user ID and password) somewhere first. If you have just entered these and then immediately see a 401 error, it means that one or both of your user ID and password were invalid for whatever reason (entered incorrectly, user ID suspended etc.).
What you really need to do is to supply Credentials to the WebClient something like shown below.
WebClient client = new WebClient();
client.UseDefaultCredentials = true;
client.Credentials = new NetworkCredential("username", "password");
If your app is running behind a authenticating proxy then you may want to supply the proxy server with credentials as shown below
WebClient client = new WebClient();
WebProxy wp = new WebProxy("PROXY_ID_OR_HOSTNAME", PROXY_PORT);
wp.Credentials = new NetworkCredential("NETWORK USER ID", "PASSWORD", "DOMAIN");
client.Proxy = wp;
client.DownloadFile("http://www.monsoondata.org/wx/changfs.png", "F:\\changfs.png");
Regards,