In other cases we had with the NTVDM error, it was caused by the antivirus:
http://www.fsdreamteam.com/forum/index.php?topic=2284.0http://www.fsdreamteam.com/forum/index.php?topic=2573.0It's either that, or you have some a "Compatibility" mode for FSX active. To fix this, you need to right click on the FSX.EXE file, select "Properties", select "Compatibility", and uncheck all the options, which is the default setting anyway.
Another possible explanation, the antivirus has corrupted the file trying to "heal" it, before you excluded it.
Note that, it's not enough to exclude the file from scanning in the FSX folder. Before is installed there, it's downloaded and uncompressed in the Windows temporary folder which means, you have to download and install with the antivirus entirely shut off, otherwise the file might be blocked and/or corrupted before being copied in the FSX folder.