Windows 8 comes with Hyper-V pre-installed.  If you want to run Virtual Box, you must completely uninstall Hyper-V through the “Add and Remove Programs” option from the Control Panel.  Failing doing so will result in:

  • Virtual Box will display error messages when starting up your Virtual Machine
  • You will not be able to change the number of processors in Virtual Box
  • You virtual machine will run very slowly because of the inability to increase the number of processors
  • Hardware acceleration will be disabled

Stopping the Hyper-V service is not enough.