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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

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2  
Do you have the most recent version of your motherboard's BIOS? –  Breakthrough Nov 5 '12 at 19:47
    
Updated it 2 hours ago, it still says that VX-T is enabled but I can't see it in Windows –  StepTNT Nov 5 '12 at 20:22
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2 Answers

up vote 15 down vote accepted

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.

Goto -> Control Panel -> Programs and Feature-> turn windows feature On and Off

and then uncheck "hyper v"

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Are you saying that there's some kind of bug that stops me from having both Hyper-V and Virtualbox running? That's weird! I'll try to see if your solution works –  StepTNT Nov 20 '12 at 18:37
    
weird but real. It worked for me too –  David Apr 18 '13 at 20:05
4  
This isn't a bug - it's just a consequence of how Windows 8 runs on HyperV. When you enable the HyperV feature, you actually install HyperV as the host OS (hypervisor) and change your existing Windows 8 installation into a special (guest) OS running on HyperV. –  Jeff Meatball Yang Jul 6 '13 at 5:29
    
This also solved the problem for me. I feel like this should be made more obvious in Windows 8 when installing the hyper-V option. Alternately, I think the VirtualBox guys could be more clear about this potential edge case in their manual where they discuss enabling VT-x –  Chris Aug 26 '13 at 20:05
    
Oh my god, spend last 4 days looking for an answer of this strange behavour. I must have enabled Hyper-V and then forgot about it. I was trying to figure out what caused this problem. Thank you! I've also had problems with internet connectivity recently. I'll research whether Hyper-V with VirtualBox clash caused that as well. –  Michael Cwienczek Sep 3 '13 at 14:19
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I found this article/post on Intel's site. Given that for this person, a clean install of Windows 8 (ignoring the fact that this was before the proper release,) I would suggest doing the same.

I have seen no end of problems lately for people who upgraded, which a clean install fixed.

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It's a clean setup –  StepTNT Nov 5 '12 at 20:23
    
@StepTNT - Its entirely possible the reason you have this problem is you updated the BIOS revision. I would do a clean install just to rule it out. –  Ramhound Nov 5 '12 at 21:06
    
I've got this problem before updating the BIOS, and I've updated it to try to fix it –  StepTNT Nov 5 '12 at 21:11
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