A few days ago I posted the solution to the error 0x000000C4 that you’ll run into if you try to install Windows Server 2012 R2 on VirtualBox. Now I just noticed that you get the same error message when you install Windows 8.1 Preview on VrtualBox:
Your PC needs to restart.
Please hold down the power button.
Error code: 0x000000C4
The solution to the problem is the same as for Windows Server 2012 R2: