It is important to understand the distinction between Windows IIS Web server permissions and Windows NT/2000 NTFS (NT file system) permissions. Unlike NTFS permissions, web server permissions apply to all users accessing a web site. NTFS permissions apply only to a specific user or group of users with a valid Windows account. NTFS controls access to physical directories on the server, whereas web permissions control access to the virtual directories comprising the web site.
Web access permissions use the Windows account IUSR_computername, where "computername" is the name of the server on which IIS is running. When users access a site using anonymous authentication, they use this account. By default, IUSR_computername is given the "Guest" group's NTFS permissions by IIS for the actual folders that comprise the web site. These permissions can, however, be changed for any folder or file in a site. For example, web server permissions can be used to control whether users visiting a web site are allowed to view a particular page, upload information, or run scripts and executables on the site. NTFS permissions can be applied to prevent ordinary users from accessing private files and directories.
Note: Disabling web server permissions in IIS, such as disabling the Read permission, restricts all users from viewing a file, regardless of the NTFS permissions applied to those users' accounts. Enabling Read permission in IIS will allow all users to view that file, unless NTFS permissions that restrict access have been applied. If both web server and NTFS permission are set, the permissions that explicitly deny access take precedence over permissions that grant access. If web permissions and NTFS permissions differ for a directory or file, the more restrictive settings are used.