NTVDM stands for "NT Virtual DOS Machine". It is the Win16-bit subsystem that runs under Windows NT, 2000, and XP to allow you to run 16-bit apps on a 32-bit operating system. Also known as WOW - Windows on Windows.
NTVDM itself is a legitimate part of the Windows O/S - but it is possible that whatever is invoking it on your PC is not.
Most likely the culprit is not an installed program but an executable that loads itself (or tries to) at startup, so will be in your Startup folder (remember, there can be several startup folders...) or in the registry under HKLM\Software\Microsoft\Windows\Current_version\Run.
Could be a virus, trojan or other malware.
SD