jquery ajax with asp.net not working

Thank you @DaveWard and @CodeRoller for your help which led to this solution:

1. Install ASP.NET Ajax Extensions (HERE)
2. Add reference to System.Web.Extensions.dll
3. Modify web.config to include the ScriptModule httpModule (see below)

My jQuery ajax call and the WebMethod are still the same as my original post.

My web.config has been modified as below:

  <system.web>
    <compilation debug="true">
      <assemblies>
        <!-- A bunch of other assemblies here-->
        <add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
        <add assembly="System.Web.Extensions.Design, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
      </assemblies>
    </compilation>
    <httpModules>
      <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
    </httpModules>
  </system.web>

I hope someone else will find this useful.