Your PC needs to restart.The solution is here: http://4sysops.com/forums/topic/windows-server-2012-r2-on-virtual-box-error-0x000000c4/ but it needs to be tweaked slightly for OS X.
Please hold down the power button.
Error code: 0x000000C4
Open the terminal and do the following, making sure to use the name of your Win 8 vm instead of [name]
cd VirtualBox\ VMs
VBoxManage setextradata [name] VBoxInternal/CPUM/CMPXCHG16B 1That's it!