trouble converting a web service to respond to AJAX requests

 I am attempting to set up a web service that I have created so it will respond to AJAX requests from a web application I am writing. After watching the excellent video here:
http://download.microsoft.com/download/f/7/a/f7adf2b4-6554-4027-bac6-c853bc0a4f9d/HDI-AJAX-WSAddAjax.wmv
 I thought I had everything set up correctly but, I am still getting this error:

 Only Web services with a [ScriptService] attribute on the class definition
can be called from script.
 But, I have included the [ScriptService] attribute in my code. It doesn't seem to be working, though.
 Here is my code:
 using System;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.ComponentModel;
using System.Security.Cryptography.X509Certificates;< ...