I was installing Windows Server 2012 R2 64-bit to VirtualBox 4.3.6. I loaded an ISO to the virtual DVD drive and started the VM. After showing "Loading files" progress, my VM displayed error 0x000000C4 and VirtualBox popped up a dialog to confirm shutting down my VM.
Open a command prompt and change the directory to where VirtualBox has been installed, for example, "C:\Program Files\Oracle\VirtualBox". Execute the following command line:
VBoxManage.exe setextradata w12r2 VBoxInternal/CPUM/CMPXCHG16B 1
w12r2 is my VM name. If your VM name contains space, please use quotes.