In fact, MS-DOS was run in an emulator in every version of NT, as long as it was supported anyhow. The NT kernel effort was lead by Dave Cutler, who came out of the VMS development group at DEC. The NT kernel was always capable of running multiple OS stacks simultaneously. Everyone knows the Win32 API, and now as well the tablet centric WinRT API, but NT operating systems have always hosted a POSIX (portable UNIX) personality as well (used primarily for shell stuff by IT guys, and sometimes to qualify as an acceptable OS on publicly funded projects).