I've had the same issue. I put a crossfire card in my Alienware M17X R2 and when I installed the drivers, it added this AMD HD Audio device that cause conflicts with my laptops sound card that caused the same issue, not booting into Windows with the drivers active. Safemode worked perfect. Once I disabled the AMD HD Audio device in Device Manager, everything has been peachy.
Edit: Although, Windows Update keeps re-enabling the HD Audio device that is causing the conflict.