simple web service doesn't work on server

Hi,
The following 'hello world' webservice works fine on my computer (VS2008) but I can't get it to work on the server--either from an aspx page (below) or when invoked directly.  I get '404 page not found' on the direct invocation. 
<%@ WebService Language="VB" %>

Imports System.Web
Imports System.Web.Services
Imports System.Web.Services.Protocols

' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()> _
<WebService(Namespace:="http://tempuri.org/")> _
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
Public Class ws3
Inherits System.Web.Services.WebService

<WebMethod()> _
Public Function HelloWorld() As String
Return "Hello World"
End Functio ...