The Anonymous User Account, which is available whenever IIS is installed, allows users access to files and folders without supplying network login credentials.
Under Windows Server 2003 (IIS 6), the Anonymous User Account has the following naming convention: IUSR_<MachineName>, where <MachineName> is the actual name of the webserver on which the account is located. For example, if IIS was installed onto a server named Blackbaud, then the Anonymous User Account would be named IUSR_Blackbaud.
Windows Server 2008 (IIS 7) has replaced this account and naming convention with the IUSR system account. This account is named IUSR regardless of the computer name where it is located.