Intel VT-x disappeared after upgrading to Windows 8

I've upgraded to Windows 8 some weeks ago (MSDNAA!) and today, while I was trying to run a VM, I've noticed that VT-x is disabled.

Now, Hyper-V works without problems, as I've tried emulating Win7 and WP8; but VirtualBox says that VT-x is disabled and it can't run my VM.

Just to double check, I've run Securable by GRC and the Intel utility and both says that I've got VT-x off.

Here comes the problem:
VT-x is ON in my BIOS, and it worked flawlessly before upgrading to Windows 8!

Any kind of suggestion?

EDIT: My CPU supports VT-x: http://ark.intel.com/products/52219

Solution 1:

Had same issue. I enabled the Hyper-V to test the built-in virtualization in Windows 8 and that caused the OS to report Vt-x incorrectly.

I disabled Hyper-V from the Windows features and now VT-x is enabled again.

Go to Control Panel → Programs and Features → Turn Windows features On or Off and then uncheck Hyper-V.

Solution 2:

First, check if VT-x enabled in BIOS\UEFI Second, disable Hyper-V:

dism.exe /Online /Disable-Feature:Microsoft-Hyper-V

Here is detailed instruction: http://druss.co/2015/06/fix-vt-x-is-not-available-verr_vmx_no_vmx-in-virtualbox/