How does Windows hide internal users?
Solution 1:
My goal is to make it possible to login in as NT Authority\SYSTEM from the welcome screen. How do I do that and where has windows protected this possibility?
You won't be able to do this. The account is not an interactive account. See here: http://msdn.microsoft.com/en-us/library/windows/desktop/ms684190%28v=vs.85%29.aspx