I just tried to run VirtualBox on my Windows 8 computer where I already had activated Hyper-V. That was a 'no go' - it borked out with missing VT-x support: You simply can't run Hyper-V and VirtualBox simultaneously, as any application that uses the virtualization-specific instruction sets on the CPU (Intel VT-x in my case) does so alone and therefore locks other programs out of using them. Too bad, but probably security-wisely good.
Links, commands and settings I someday probably won't find again