This is a very quick tip for those trying to Get Windows 8.1 running in Virtualbox
If you get an error
Your PC needs to restart.
Please hold down the power button.
Error code: 0x000000C4
The solution to the problem is this command:
You have to replace vname with the name of your Virtual Machine which you can find in the VirtualBox interface or with this command:
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe"list vms
Don’t forget to set the VM name in quotes if it contains blanks.
The command enables the CPU instruction CMPXCHG16B. More info here.
I tried it with the VHD of Windows Server 2012 R2 Preview and it with the ISO.