When you start Windows 8.1 setup from a DVD or iso file in a VirtualBox VM, you may get the following error:
This PC can’t run Windows 8.1 Preview
You can’t install Windows 8.1 Preview because your processor doesn’t support CompareExchange128.
Run the following command at the command prompt of the host computer
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" setextradata [vmname] VBoxInternal/CPUM/CMPXCHG16B 1
Replace the [vmname] with the name of the virtual machine.
Happy system administration.