After updating my vs2008 with silverlight, i had problem to run my old aspx.cs files

b4 i updating....when i open the cs file there will be......
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

after i update with silverlight....only these exist....the rest gone
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

and when i run the old file after updating with silverlight....this error appear
Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.

Source Error:

Line 47: ASP.NET to identify an incoming user. Line 48: -->Line 49: <authentication mode="Windows"/>Line 50: <!--Line 51: The <customErrors> section enables configuration
Source File: J:\Advance Diploma\AACS4134\Practicals\Practical 4A\practical4a\web.config Line: 49


Version Information: Microsoft .NET Framework Version:2.0.50727.3053; ASP.NET Version:2.0.50727.3053

What should i do???