• You install Windows 8.
• You change the Hosts file by specifying custom IP-address-to-host-name mappings to prevent users from browsing to some websites.
• You run a scan in Microsoft Windows Defender.
In this scenario, the Hosts file is detected as a SettingsModifier:Win32/PossibleHostsFileHijack by Windows Defender.
See the KB for instructions how to exclude your Hosts file from being scanned and trashed.