You need to set the User-Agent HTTP header for your request. You can easily do this as follows (there is a property which directly supports this):

objMyRequest.Us erAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)";

where objMyRequest is of type HttpWebRequest.

Set this before you write to the request stream or get the response. I can highly recommend...