On our webservers (Windows Server 2008 R2 machines with IIS 7.5), our application pools run using the "ASP.NET v4.0" account instead of the NetworkService account.
But we had some trouble assigning permissions to this ASP.NET v4.0 account. It does not appear as a local user on the machine. When we browse for local users, groups, and built-in accounts, it does not appear in the list.
Luckily you can still add the user through the NTFS GUI by typing it in directly. The name is in the format of IIS APPPOOL{app pool name}. For example: IIS APPPOOL\ASP.NET v4.0.