“Your PC need to restart, Please hold down the power button Error Code : 0x000000C4”.
Here is a solution I found at https://www.virtualbox.org/ticket/10792:
Open terminal windows and execute this command:
VBoxManage setextradata [vmname] VBoxInternal/CPUM/CMPXCHG16B 1
where [vmname] is your Windows 8.1 virtual machine name.
Here is my screenshot: Install Visual Studio 2013 Preview on Windows 8.1 Preview
Update on July 10: You need to the Oracle VirtualBox 4.2.16 or later to install VirtualBox Addition in order to make the full screen, shared folder and other features.