You can use this code
using (WebClient web = new WebClient())
{
web.Headers["User-Agent"] =
"Mozilla/4.0 (Compatible; Windows NT 5.1; MSIE 6.0) " +
"(compatible; MSIE 6.0; Windows NT 5.1; " +
".NET CLR 1.1.4322; .NET CLR 2.0.50727)";
}