When you install Windows Server 2012 R2 preview, you may get the following error,
Your PC needs to restart.
Please hold down the power button.
Error code: 0x000000C4
The solution of the problem is straight forward, run the following command on the host computer:
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" setextradata [vmname] VBoxInternal/CPUM/CMPXCHG16B 1
Replace [vmname] with the name of your machine.