Ed Bott provides step-by-step instructions on how to boot Windows 8 from a virtual hard disk.
Note: This will only work on a PC running Windows 7 Ultimate or Enterprise (preferably 64-bit). The Boot from VHD feature does not work on other versions of Windows 7. Also, if you’re using BitLocker encryption, as Ed said, "stop right now. This will just make you cry."
How to use a VHD to dual-boot Windows 8 on a Windows 7 PC | ZDNet
Note: This will only work on a PC running Windows 7 Ultimate or Enterprise (preferably 64-bit). The Boot from VHD feature does not work on other versions of Windows 7. Also, if you’re using BitLocker encryption, as Ed said, "stop right now. This will just make you cry."
How to use a VHD to dual-boot Windows 8 on a Windows 7 PC | ZDNet