You are trying to access a webservice using a GET request. First of all
you must have a way to define a "default method" because this way the
webservice does not understand what method to call. As far as I know,
such a mechanism does not exist.
In order to properly execute webservice's method, you need to send via
a POST method a proper SOAP request, declaring the wanted method and
the parameters you want to pass.
If you want to call a webservice from the browser you have to post a
form with the appropriate SOAP request.
Otherwise you can cirvent this and create an aspx page that handles
the webservice call and returns the XML via Response.Write. After the
XML is written you can call Response.End to stop processing.