A few things that happened to me that in the end were either one of these fixes:
1. You chose the COM Port wrong, then this happens. use regedit and search for Nissan Datascan and change the COM port to the one used by the serial to USB Adapter.
2. uninstall NDS completely and clean all traces of it, then reinstall and dont get the port wrong again.
3. if on win7, either use Virtual XP or right-click start NDS with "run as an administrator"