I reinstalled Garmin Mobile PC (numerous times now!), and before I upgraded to 5.00.70g I ran Mobile PC (OK), and found two user-defined tracks in there. Deleted them. "Ran" the Mobile PC upgrade again, and the same problem was back.
On the good news front, I'd forgotten that because this laptop is new, Windows by default hides "hidden" folders, which is why I couldn't find that GARMIN_STRK.nvm. I've now found the Garmin application data folder and deleted it entirely.
I've installed Mobile PC from DVD again, upgraded to latest version. Reinstalled the Australian maps (and downloaded and reinstalled the free map upgrade), unlocked everything etc. I've downloaded (again) Garmin Plugin Communicator, and Garmin POIloader and reinstalled all my custom POI sets. Just have to put my "favorites" back in.
IT ALL WORKS! So the problem was the application data folder. Interestingly, once I understood this, I deleted the GARMIN_STRK.nvm file from inside the folder and "ran" Mpbile PC again, and the .nvm file reappeared, along with the error. So the whole folder had to be deleted.
Thanks for all the help on this. Apart from losing some sleep last night (!) and some hours sitting in front of the laptop, I guess at least I'm confident about reinstalling everything again from scratch now.
However, once bitted twice shy in terms of transferring tracks into Mobile PC. It would be nice, but I can live without them...
Bandicoot