I already tried that, too.

I booted from the disc, but the system disc for Windows 7 offered no repair options.
It only offered the option of doing a new install of  the operating system, which would overwrite the hard disk. 

I don't want to lose all my unbacked data on a perfectly good hard disk when the only thing wrong on the entire setup is one stupid registry setting involving the video card that makes the entire desktop unseeable.

I may have to do this, but if I do, the entire thing will have been ten times the agony it should have been.

I'm taking the machine in for service now, hoping that techs there have other ideas.
Word of advice to all :  Do NOT believe windows when it says you can easily revert to a restore point.  Obviously you can't, if your rollback has glitched the registry somehow.

I'm still really angry at DAZ, NVIDIA and Microsoft.
And DAZ still has not replied to my original request for help, which is now almost a moot point.
Eventually if I get Windows restored, I still will have a Carrara issue.