Code 43, graphics card no longer working

So I've read numerous similar threads on here, but none of these seem to have resulted in any fix, so I thought I'd try.

Without warning, and for no apparent reason, my graphics card is no longer running correctly, I get an code 43 error. I haven't installed any new hardware or major software recently, bar a couple of games on Steam (TR3 and Thief Gold). I'm running two nvidia GeForce 9800GTX cards in SLI, both are reporting code 43 errors.

The problem first started while running Thief Gold on Steam, game glitched out, computer crashed, and on reboot problem started. Symptoms of the problem include multiple blue dot artifacts on startup and in BIOS screen, screen resolution stuck at 1600x1200 (usually have it at 1920x1200), desktop icons randomly rearranged, no 3D applications (games, video editing software) seem to run.

Unhelpfully, all nvidia software (control panel etc) is also failing to start now, so I can't at the moment look at the running temperatures of either card.

Brief PC specs;

PSU - CoolerMaster 700W

CPU - Intel Core2 Quad Q6700 2.67GHz

RAM - 4GB

Operating System - Windows Vista 64bit Home Premium Service Pack 2

GPU - 2 x Nvidia GeForce 9800GTX in SLI

Steps I've already taken to fix the problem (IMPORTANT - none have worked);

1. Roll back system to a point well before problem developed.
2. Roll back GPU driver to previous versions that I know haven't given any problems.
3. Update driver (both automatically in Device Manager, automatically on the nvidia website, and manually from the nvidia website).
4. Uninstall and reinstall driver.
5. Uninstall and reinstall GPUs.
6. Check BIOS settings (had heard that one solution may be to modify value of "AGP aperture size", but can't find such an option).
7. Take apart rig, remove GPUs, try both individually and in different PCIe slots.
8. Removed processor, cleaned, repositioned, also cleaned all fans, cleaned out rig.
9. Removed GPUs, cleaned, checked that fans are working.

I'm fairly certain that this ISN'T a hardware issue, since the problem manifested itself at first sporadically, and only after a day or so has become consistent (all boots now result in this problem).

My thinking so far;

a) Could a rogue Windows update have triggered this? This is the only possible change that could have occured between the last time the computer ran fine, and when the problem arose.

b) Could the PSU be failing? I've never had a problem in this regard before, and no other obvious symptoms (random crashing etc) seem to be present...

c) Is it a GPU hardware fault after all? Not mad keen about baking either card, but if needs must...

Any help would be massively appreciated! Cheers :)