I'm working on my first ASP.NET application in Visual Studio 2008 running on a 2008 Server. It's a simple thing, just designed to get some information from our users. I did need them to authenticate themselves against Active Directory prior to visiting the webpage. I turned on Windows Authentication and changed the security settings on the folder my application is in to allow only domain users and I removed the IIS_IUSRS user permission from the folder (on the assumption that I didn't want non-authenticated users to have access). Well, turns out I was wrong. That just generated an error on the site. Adding IIS_IUSRS back solved that problem.