It is entirely possible in Windows NT and beyond to remove all admin accounts and lock the system down. This is not a bug or design flaw.

I have 2 methods I use before we slip in a new hard disk (pull the old one to keep the files around) and here they are. No offer to write tutorials on use.

1. NTPASSWD.
2. Just after the gaffe do not safely shutdown. Press the reset button and select LKGC (see google.com)