You need to add this code in web.config to enable calling WebMethods from javascript
<system.web>
<httpmodules>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
</httpmodules>
</system.web>
Edited:
I missed the exact version for the ScriptHandler
For VS 2008. You need to use this
<httpModules>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
</httpModules>