New
#11
I meant Device Manager was just showing the generic VGA driver, not the Quadro 4000, so it couldn't find the appropriate driver itself. Anyway, I downloaded it from Nvidia and it installed fine.
The overall problem seems to have been caused by SpySweeper which I uninstalled prior to trying the Repair Install. When I uninstalled it, I realized I had installed it around the time this problem appeared, so I just rebooted rather than start the repair and everything's worked fine since, now after two or three reboots. I'm also running Norton 360 so SpySweeper was largely redundant anyway.
I'm keeping my fingers crossed that everything's stable again.