DNN (formerly DotNetNuke) recently started requiring that a website be ran with it's own AppPool user.  

These steps assume you have already added a new AppPool in IIS, which will create a new AppPool identity (user).

1. Once everything is configured in IIS with a new AppPool, open the folder of the website with windows explorer
2. Right click in this folder, and choose properties.
3. Open the security tab.  Click the Edit button below the list of usernames.
4. Click add, then select your local computer for the location,
5. In the textbox for object names, enter "IIS AppPool\<username>"  (without quotes) where <username> is replaced with your AppPool user name (identity).  
6. Hit the button labeled "check name."  (If everything was set up correctly, it should resolve the name.)  
7. Then click okay to add it to the list.
8. Set the permissions to allow this user full control of the directory.

This article is short and may not be very helpful, but I can't ever seem to remember the syntax for adding an AppPool user to a folder.