I third the dummy plug idea. Also, disable you onboard graphics. Speaking from my experience with my rigs that each use 2 MSI Frozor III 7950's, unless I disable my onboard graphics, and use dummy plugs, one on each video card, I get BSOD's every time my miner starts, and that's with the latest greatest driver from AMD, and the latest version of cgminer as of 05/07/13.

Hope it helps!