If you try to start using an iso file Windows 8.1, you'll get the error code: 0x000000C4

On command line (for me linux)

myuser@linux:~> /usr/bin/VBoxManage list vms
"centos7" {810ce7a4-6018-4ca2-abd6-29238b2c8d41}
"windows8" {3615a9f3-fb0a-45ac-b1ad-99853f4ee09f}
myuser@linux:~> /usr/bin/VBoxManage setextradata windows8 VBoxInternal/CPUM/CMPXCHG16B 1

Then start again your VM.