virtualbox - Why Virtual Box won't give me option to create 64 bits guests?

My host is x64 bits Windows 8.1.

I downloaded the latest Virtual Box (4.3) and I'm trying to create a VM with a 64 bits Ubuntu OS (ubuntu-12.04.3-desktop-amd64).

When I go to New VM wizard, it doesn't give me option to select "Ubuntu (x64)" as I have seen in other people's screenshots, only just "Ubuntu". As a result, the ISO can't boot. I tried in another PC and Virtual Box gives the x64 variants to most listed OS...

Control Panel shows x64 OS, x64 processor. My host laptop is a Sony Vaio VPCZ22UGX/N, Intel® Core™ i7-2640M processor. CPUz shows Vx-t is available on my processor, of course.

Here is what I tried so far:

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  I enabled IO APIC as required in the docs.

  
  


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  I have virtualization enabled in the BIOS. It works fine in VMware.

  
  


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  Check that Hyper-V is not running or even installed on my Windows. Same for VMware.

  
  


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  I also tried running the command:

  
  

  VBoxManage modifyvm [vmname] --longmode on

  
  

for that VM, but no change.. I think the issue is really that I can't select x64 variant of the Ubuntu OS for that VM. Other people seem to indicate that's a requirement, but I don't get that option for some reason.

I spent a lot of time and can't find what's wrong... Anyone knows what could be missing here?

Thank you very much!!

Eduardo