Migrating app in IIS 6 to 7.0

I have web app written in .NET 2.0 hosted under IIS 6.0 with integrated authorization enabled. I moved this app to Windows Server 2008 R2 with IIS 7.5 with Windows auth enabled, and now I am getting this error. Any help?

Security Exception

Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.

Exception Details: System.Runtime.InteropServices.COMException: An operations error occurred.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[COMException (0x80072020): An operations error occurred.]
   System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail) +557
   System.DirectoryServices.DirectoryEntry.Bind() +44
   System.DirectoryServices.DirectoryEntry.get_AdsObject() +42
   System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne) +98
   System.DirectoryServices.DirectorySearcher.FindOne() +49


[SecurityException: Error authenticating user.]
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +80
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +171

Is it possible that your new environment has a trust level other than FullTrust? What happens if you place this under the <system.web> section in the web.config?

<trust level="Full" originUrl="" />