I have been struggling with some ASP AJAX code.  I moved the site from Server 2003 32-bit IIS6 to IIS7 on Server 2008 64-bit.  After a lot of searching and banging my head against the wall I found a blog on MSDN that told me how to add a binding redirect to my web config file so that the code would use the AJAX built into .NET 3.5.  Here it is. 
<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
Obtained from: http://blogs.msdn.com/webdevtools/archive/2007/07/28/upgrading-asp-net-ajax-1-0-websites-and-web-applications-to-net-framework-3-5.aspx