We might have it working as of today, or at least cut back considerably as we didn't see it in 2 hours and were able to reproduce it in a matter of minutes previously (on a realestate.com.au web page with all sorts of flash ads, just scroll the mouse and slide it around the page).

We've built 15+ systems with the Asus EN9500GT Magic (512MB) video card. This is the only one that errored. After doing most of what you mentioned (except check the 12v rails as I'm sure that's not a problem with a Corsair VX450w, and Prime95 wouldn't prove anything either way other than the system is stable or also has other problems) and a few more fixes that I've seen scattered around the internet (underclocked 1333MHz memory to 1066MHz, changed the Power Management Mode in 3D settings to "Prefer Max Performance", make sure BIOS was set to Plug and Play OS: Yes (actually wasn't included in this BIOS), removing update kb952287, and trying 1 stick of RAM) with nothing solving the problem.

I put everything back to the way it was, then I uninstalled NV drivers and swapped the video card with an ATI 4350, and ran Ccleaner to rid the system of as much NV remains as possible automatically upon booting up again. It ran fine for an hour, so I got brave. I reinstalled his original video card with the original drivers and again ran Ccleaner to rid the system of as much ATI remains as possible automatically. Then we surfed the web, and specifically the realestate.com.au website (as that's where this customer believes they have the most problems and we fairly easily reproduced it earlier). 2 hours without a single crash. At this point I have my fingers crossed that its gone, or slowed down to almost gone.

Everything I've seen is either a work around, or the only way of really getting rid of it was by changing to a video card that uses a different driver. All the more reason that Windows corrupting the drivers makes sense to me (which might well effect all versions of the same driver family for same or similar cards). But again, I won't be sure this is gone until they run it for a few weeks without the error.