You might find your virtual Windows 8 complaining that "Your PC needs to be repaired". It says you have to "Press Enter to try again", or "Press F8 for alternate boot options". But none of them work.
You might try this way:
Close the Windows 8 window, select your virtual machine in VirtualBox, click Settings > System > Processor and check the "Enable PAE/NX" box. Click OK, and restart.