In order to install and use Windows 8.1 preview/Windows Server 2012 R2 preview in VirtualBox, you have to run this command:
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" setextradata "VM Name" VBoxInternal/CPUM/CMPXCHG16B 1
You can see your VM names using this command:
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" list vms
Источник: Сайт
