VT-x is not available for VirtualBox

Problem

A virtual machine running Ubuntu in VirtualBox which was working fine threw up an error when I tried to restore it. The error read:

Failed to open a session for the virtual machine Ubuntu-12.04-LTS-x64.

VT-x is not available. (VERR_VMX_NO_VMX).

Result Code: E_FAIL (0x80004005)
Component: Console
Interface: IConsole {db7ab4ca-2a3f-4183-9243-c1208da92392}

This happened even though I had not changed any settings in VirtualBox.

Solution

VT-x is hardware visualization which the Intel Core i3 M330 processor I have does support. It was strange that the hardware visualization feature suddenly was not available to VirtualBox.

Then I remembered that I had enabled Hyper-V earlier in the day. Windows 8 ships with Hyper-V and I had turned it on to try it out. It turns out that Hyper-V grabs hardware visualization and VirtualBox is left stranded not being able to access it.

I disabled Hyper-V and my VM in VirtualBox booted up without any errors.

Tried with: VirtualBox 4.2.4 and Windows 8 Pro