Im gonna guess it has to do with windows 7 and how UEFI works.
I don't know if your board has the option to run it in legacy, as legacy was phased out, and windows 7 did not support UEFI.
So i am guessing somewhere UEFI and Windows 7 is not playing nice.
Do you absolutely need windows 7?
You never toyed with the idea of playing with twins?
It only plays nice, when they are actually in sync and can tango together.
Xfire + SLi is more like twins who are out to sabotage each other and always end up stepping on each others feet with very little harmony, as they want to be the one on top and not off to the side.
This is also why Nvidia gave up on it.
There team of monkeys typing random code, could never write something that complicated and ever have it work properly.
And soon AMD saw no point either, as there competition gave up on it.