Reminds me of SLI issue I had after I changed my motherboard. I had the SLI option not appearing and when it did appear and I enabled it.. the Nvidia drivers would crash immediately and require reinstall. Asus support didn't know what the problem was but there was some rumour on a forum that the cause was hacked/pirate windows. At the time I had been using just that and so I downloaded a legit trial windows 7 from microsoft and was amazed to find my SLI worked perfectly!
If you're using non-pirate windows then obviously your problem is caused by something else.
Have you tried a different install of windows? Always a good idea to have a second install available for testing.