I can't work with my database and IIS7

 I have written an asp.net 2.0 app that run on iis7 and windows vista.
When my code tries to access the database I get the following error :
Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.



Description: An
unhandled exception occurred during the execution of the current web
request. Please review the stack trace for more information about the
error and where it originated in the code.



Exception Details: System.Data.SqlClient.SqlException: Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.

 
Here is the class code :
 
using System;
using System.Data;
using System.Configuration;
using System.Data.Commo ...