pinholesteve:

Thanks for the quick reply Saby.

I've been trying a pretty heavy-handed approach I've seen mentioned elsewhere - to basically remove (or rename) the Pinnacle-related folders under appdata\local from the problematic account.  When trying to start pinnacle after that, I can see the folders get rebuilt, but still no success in starting PSU (nothing after the splash screen.)

I tried your suggestion, of just removing the 23.0 folder to something else, and similarly, the next startup attempt rebuilds 23.0 and its subfolders & their contents, but still no joy after the spash screen.

Any other thoughts? 

I think if it was me I'd do a complete uninstall, clean the registry and reinstall in your main profile from scratch

Out of interest hgow did you do the upgrade from 8 to 10