In Windows 7 and Windows 2008 R2 (IIS 7.5), the default identity for application pools is set to ApplicationPoolIdentiy.
ApplicationPoolIdentiy specifies the service account that is used when you create an application pool. The service account is the same name as the service account. So if you created MyNewAppPool, the ApplicationPoolIdentiy is IIS AppPool\MyNewAppPool. You won’t be able to find this service account if you just browse or search for “MyNewAppPool” alone. You MUST specify IIS AppPool\MyNewAppPool.
Previously in older versions of IIS, it was set to “NT Service\NetworkService.” You can now set the ACL for “IIS AppPool\<app pool name>” – the user account created for each new application pool.
This is a great article about Application Pool Identities in IIS7