I found that the WebClient kept removing my User-Agent header after one request and I was tired of setting it each time. I used a hack to set the User-Agent permanently by making my own custom WebClient and overriding the GetWebRequest method. Hope this helps.
public class CustomWebClient : WebClient
{
public CustomWebClient(){}
protected override WebRequest GetWebRequest(Uri address)
{
var request = base.GetWebRequest(address) as HttpWebRequest;
request.UserAgent="Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; WOW64; Trident/6.0;)";
//... your other custom code...
return request;
}
}