[Saruman]

Windows NT Virtual DOS Machine – Windows NT4/2000/XP/2003. Whenever you start a DOS or 16-bit Windows program (Windows 3.1 software, or a program designed for Win9x/NT4 but which has a mixture of 32-bit and 16-bit programming code) under Windows NT4/2000/XP/2003, that 16-bit program gets run by the Windows NT Virtual DOS Machine, NTVDM, which basically emulates a DOS/16-bit environment so that your program can run. There will be a different instance of NTVDM running for each DOS/16-bit program that you run. (Note that NTVDM in turns starts a different process called WOWEXEC).

Recommendation :
If NTVDM is in your Task List, you have a DOS or 16-bit program running. If it is twice in your Task List, you have two DOS or 16-bit programs running, etc... Leave alone; as soon as you close your running DOS/16-bit program(s), NTVDM will disappear from your Task List under Windows 2000/XP/2003 although it will remain in your Task List under Windows NT4 – under NT4 this is by design to ensure that the next 16-bit program you run loads faster.