OK, further research has identified that the system is displaying all the characteristics of a fresh install because that's what's happened!
Machine was powered down properly and without apparent problems. But it seems that my son (who's PC it is) has somehow been presented on startup with some sort of "Reinstall WIndows?" option and must have selected Yes, Unattended. Being autistic, he's not able enough to understand what this means, nor regrettably able to explain things clearly, and it looks like he has now obliterated all trace of the previous installation. Including all the Restore points.
So my hope now is that I can finish the reinstall, then try to recover some of the data files. Looks like I'm going to be busy.
Any ideas what might have caused the option for an unattended reinstall to appear? I've got a lovely big logfile in the \Windows\Panther directory that shows what happened, but not why.