It is a good security policy to disable optional, unused sub-systems Microsoft
included with Windows - NT, W2K, & XP. To disable the obsolete and insecure
posix and os/2 subsystems by clearing the multistring value named "Optional" in the Registry entry:
Under some OSs the string with have both "os/2" and "posix" strings. XP
generally just has "posix". In any case, clear the value.