Hi SD! Thank you for having a look and a think.
SWMBO is running the Windows firewall and only one anti-virus (Avast). She does insist on running Google Sync at all times and Mozibackup likewise. Just as an attempt at good practice, the computer and its power supply are on stilts to improve cooling and we have noticed around a five to ten degrees drop in temperature when the computer is working. It should be said though that it was at no stage redlining. Even at the height of summer here the hottest bit, the HDD was only at 48 to 56ºC and now that it's about 8º cooler indoors it seldom show as significantly above 50º. Last night when working hard the Motherboard was at 37º, the Graphics card at 38º and the HDD at 51º.
Because the machine has been freezing fairly frequently over the past couple of weeks I decided last night to bring through my external hard drive to do a full back up and image, independently from her own arrangements (see above). Within twenty minutes the job was about 30% completed which I thought was pretty fair so I tripped off to bed. This morning it had only got as far as 50% or so when my wife started work again at 0800 hrs. At 1126 hrs. when the big crash came my backup had only progressed indiscernibly from where it had been at 0800 hrs.
The machine sat unchanged, but still whirring away, with the clock frozen showing 1126 hrs. until I did a power button turn off at about 1700 hrs. Then it rebooted and the first screen recommended a normal Windows start which went smoothly. My abacus never gave me problems like this....