Call webservice using javascript and Scriptmanager not working

 
 
 
                               Configuration : Windows Vista,VS 2005,Ajax v1.0.61025
 
My Html Page
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Untitled Page</title>
    <script language="javascript" type="text/javascript">
    function fnCall()
    {
        try
        {
          var ans= Sim.Service.HelloWorld(OnMethodSucceeded, OnMethodFailed);
        }
    ...