What you need to use is the WebRequest object.  Here's a funciton that I wrote
to POST a request (watch for wrapping in this message).  This function will
return the resposne content as a string which you can load into an XMLDocument
object and parse, query via XPath, or transform using XSLT.  If the web service
uses SOAP and has a WSDL there is  nice capability in VS.NET (or even the free
Webmatrix tool) that will create a class for accessing the web service.  BTW,
the params argument need to be in the format of that of a normal form POST
request (i.e. &field1=somevalue&field2=someothervalue&field3=...)

        Public Function webFormPost(ByVal myUri As String, ByVal params As
String, ByRef errors As String) As String

            Dim retString As String
            Dim request As HttpWebRequest = CType(WebRequest.Create(myUri),
HttpWebRequest)
            request.Method = "POST"
            request.ContentLength = len(params)
            request.ContentType = "application/x-www-form-urlencoded"

            Dim writer As New StreamWriter(request.GetRequestStream())
            Try
                writer.write(params)
            Catch e As Exception
                errors = e.message
            Finally
                writer.Close()
            End Try

            Dim response As HttpWebResponse = request.GetResponse()
            Dim strmReader As New StreamReader(response.getResponseStream())
            retString = strmReader.ReadToEnd()
            strmReader.close()
            Return retString
        End Function