Hello
I've resolved my problem, by searching "NTVDM" keyword in forum !!!
The problem is in
"HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\VirtualDeviceDrivers" key.
In my PC, this key contained, for variable LDD, several lines : one for Norton antivirus (uninstalled !!!) and several same lines relating to AVAST.
Solution : keep only one line AVAST and delete all other lines.
NTVDM becomes OK, and 16 bits applications can run OK.
With the bad key, NTVDM was excluded from task's list running on my PC, and NTVDM was blocked by AVAST.
Thanks to the forum.