Thank you guys, for all your help so far - it means a lot.

Right - I have SpeedFan and CoreTemp running all the time, and the idle CPU temperature has been around 39 C (individual cores temp about 35 C).  At full load it rises to about 59 - 65 C (individual cores 48 - 55 C)

and it stays there stable, without spiking. Graphics card temp fluctuates between 40-44 C.  All fans seem to be working fine.

Now what really befuddles me beyond words is that this has never happened before - I can run Luxrender for 4 hours on the trot, with Photoshop in the background, without any probs - the temperatures are stable, there are no crashes, no freezes.

I've even tried to render in Vue first thing in the morning as I switch the computer on, from cold, and it still powers off the PC, just like someone uplugged the poor thing from the mains. Even if it were an overheating issue, how likely it would be to have the temps rise from 'cool ' to 'instant death' within the first three seconds of a render?

There's no anti-virus pop up before the powering off, no error screen, no warning, no hibernation - it's instant and total OFF.

I haven't tinkered with the diplay settings in Vue - I had no idea one could do that (and I still have precious little clue how to do it). How do I change them?

I thought about Pioneer - but decided against it. I already have DAZ 4 and I wanted to be able to import my posed Daz stuff into Vue for rendering. I don't think Pioneer lets me do that...:sigh: