problem when jquery call ASP.Net Webservice

Question

Here is my WebService

[WebService(Namespace = "http://www.rpsoft.com.vn/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ToolboxItem(false)]
[ScriptService]
public partial class DataFeeder : System.Web.Services.WebService
{
    [WebMethod(EnableSession = true)]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public object[] GetMatrixChallenge()
    {
        string challenge = "123";
        rsl.Add(MatrixChallenge);
        rsl.Add("");
        return rsl.ToArray();
    }

And here is my js using jquery 1.4.3

ServerProxy.GetMatrixChallenge = function(onSuccess, onError, isAsync)
{
  $.ajax({
        type: "POST",
        url: "DataFeeder.asmx/GetMatrixChallenge",
        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        async: isAsync != undefined? isAsync : true,
        success: onSuccess,
        error: onError != undefined? onError : this.DefaultOnError,
    });
}

When call webserive, the return is, and DefaultOnError was called

<?xml version="1.0" encoding="utf-8"?>

<ArrayOfAnyType xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://www.rpsoft.com.vn/"&gt;

  <anyType xsi:type="xsd:string">5D1A3B</anyType>

  <anyType xsi:type="xsd:string">abc</anyType>

</ArrayOfAnyType>

I thinks the format is wrong. Where is my mistake? I am using VS2005, ASP.Net 2.0, IIS7 on Windows7 I have tried to add

    <httpHandlers>
        <remove verb="*" path="*.asmx"/>
        <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
    </httpHandlers>

The code run well on Windows 2003 but can not run on Window7.