VS 2005 Webservice Client side

I have added weservice in client side using:

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
<Services>
<asp:ServiceReference Path="" />
</Services>
</asp:ScriptManager>

I used in my client side javascript the method of webservice:

service.MyHelloWorl();

I am getting error:

The server method failed with the following error: 'InvalidOpeationException' authentication failed.

Please suggest.