Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

I get this error NOT on default.aspx, but I do on login.aspx. why would one page act different than the other?!?!
I've installed ms AJAX: http://go.microsoft.com/fwlink/?LinkID=77296 already

page default.aspx is shown:
<%@ Page Language="VB" MasterPageFile="~/MasterPage.master" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="cphTOP" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="cphLEFT" Runat="Server">

    <asp:Button ID="Button1" runat="server" Text="Button" />
label1    <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
<br /> ...