Two different problems now:
The SLi is suggested as the faulty motherboard slot - but I've hunted out the telephone numbers for both the card manufacturer and the motherboard manufacturer. I'll check with both of them before making any wild decisions.
The new problem is a known conflict between Vista and (apparently) all nVidia cards. The *supplied* drivers don't immediately drive. I've discovered hundreds of messages online from people going back over two years, all complaining that when loading Vista they were immediately met by a BSOD (Black!! screen of death). Their *fix* was to restore to earlier, download new drivers (not all of which worked) and re-load Vista.
My problem here is that as the machine is a "new build", it doesn't have any access to the net, nor do I have a system restore point - so how can I possibly get the new drivers onto the clean hard drive?