After installing the preview with all its bells and whistles including Hyper-V for Windows Phone development(I think) I found that I got array of network errors and then the killer VT-x is not available
This is due to the Hyper-V install in VS2013, since only one virtual controller can own the VT-x and Hyper-V is nested into Windows Hyper-V takes ownership and breaks VirtualBox.
So the solution, first you have to disable Hyper-V
bcdedit /set hypervisorlaunchtype off
Now if you reboot you should be able to boot up VirtualMachine, now if you are thinking to yourself but what about if I want both, well simple you cant. You either go through this process of turning off and on, or you do what I suggest convert fully to Hyper-V.
This can be done through Disk2VHD so boot up your VirtualBox VM convert it to a VHD and send it on over to Hyper-V simple hey? Oh and if you are like me with fairly chunky VM’s then you will want to archive or delete the old one.