Windows 8.1 Preview cannot be installed and run directly in VirtualBox. It displays the following error message (some text like this):
Error Code: 0x000000C4
After you have downloaded Windows 8.1 Preview from the Microsoft Website, set up a new Virtual Machine in VirtualBox. In my case, I named the machine Windows8.1.
To install it you need to open a terminal window and type the following command:
VBoxManage setextradata "Windows8.1" VBoxInternal/CPUM/CMPXCHG16B 1 <enter>
After this, you can start VirtualBox as you normally do and run the Windows8.1 virtual machine. Windows 8.1 preview installation will now start.