By default, ASP.NET does not use impersonation, and your code runs using the ASP.NET application’s process identity. On the Microsoft® Windows Server™ 2003 operating system, ASP.NET applications run in an Internet Information Services (IIS) 6.0 application pool by default. The IIS application pool runs under the NT AUTHORITY\Network Service identity. On the Microsoft Windows® 2000 Server operating system with IIS 5.0 or on Windows Server 2003 with IIS 6.0 configured for IIS 5.0 isolation mode, ASP.NET applications run in a worker process that uses the local ASPNET account identity.
Source: http://msdn.microsoft.com/en-us/library/ms998351.aspx