I tried installing Windows 8.1 Preview in Virtual Box and received the below error: (If memory serves me correctly this was the same for Windows 8)
Run the following command and then try installing Windows 8.1 Preview again.
“c:Program FilesOracleVirtualBoxVBoxManage.exe” setextradata “VM Name” VBoxInternal/CPUM/CMPXCHG16B 1
This solved the issue for me. Found the information to the solution at this article here.