In order to use the Microsoft ASP.NET AJAX "Bridge" technology with the ASP.NET AJAX CTP, the .asbx file name extension must be mapped in IIS to the ASP.NET ISAPI DLL. You can do this in the IIS Administration tool by following these steps:
 
  1. Select the Web site to work with and edit its properties.
  2. On the Home Directory tab, click Configuration....
  3. In the Application Configuration dialog box, click Add. 
  4. In the Add/Edit Application Extension Mapping dialog box, add the .asbx extension and
     in the Executable box,browse to and select the aspnet_isapi.dll (in the .NET Framework
     installation directory for ASP.NET - you should be able to look at the mapping for the
     .aspx extension as an example).

  In addition, add the following entries to the <system.web> element in the Web.config file: 

      <buildProviders>
          <add extension=".asbx" type="Microsoft.Web.Preview.Services.BridgeBuildProvider" />
      </buildProviders>

      <httpHandlers>
          <add verb="*" path="*.asbx" type="Microsoft.Web.Script.Services.ScriptHandlerFactory"
           validate="false"/>
      </httpHandlers>