|NTVDM (NT Virtual DOS Machine) is the Windows 2k/XP built-in emulator to run legacy 16 bit applications that use the int 21h method to call system services.|
It is automatically executed when you launch a DOS application, you don't have to open it. Run any DOS/Win 3.X application to blow out NTVDM.
Be aware NTVDM has a limited support for int 21h interface, i.e. some applications (DOS games) require more sophisticated emulators, e.g. full virtual machines like MS Virtual PC or DOSBox (both free).