Well i can understand wanting to use what you've paid for, when it will have an effect.
But i can't figure out why you would want to play it with multi-gpu seeing as it runs maxed out on just one card? There will not be any noticeable difference.
Take into account all the hassle you are going through to get it working on sli, wouldn't you agree it makes more sense to just ditch one card for a while and give it a go before you ignore the game completely?...
I mean it just seems a bit strange to be complaining about something that can very easily be fixed (well i assume it will work with just the one card as it does on mine) and your reason for not doing that is, essentially, for a non existant boost to graphics.
Anyway, back to the matter at hand, it might actually be build 7100 that is causing you trouble:
http://www.nvnews.net/vbulletin/showthread.php?t=135250
http://www.sevenforums.com/graphic-c...li-7100-a.html
People say it didn't support it (back in July) but they now say the evga patch i linked you to earlier, was released, and that sorted most the issues people were having.
Sorry but i don't think i can be of anymore use. I'm currently running Windows 7 Ultimate, and the game ran flawlessly with just 1xgtx285, so i would like to suggest again that you just remove one card and enjoy the game (hopefully that will fix it...).
Best of luck.