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VirtualBox 4.3.20 doesn't have 64 bit guest options when installed on Windows 8.1 Pro 64 bit host OS. Solution found was to make sure that virtualization technology is enabled in your BIOS settings and 2nd disable Hyper-V in Windows. After having done this, virtualbox will now show 64 bit guest options when creating a virtual machine.

Virtualbox should detect whether Microsoft Hyper-V is running or not and warn the user but it doesn't. Virtualbox won't be able to provide virtualization while Windows Hyper-V service is running. To disable Hyper-V, go to Control Panel -> Programs and Features. Click on "Turn Windows features on or off" (left hand side). Look for Hyper-V and un-select it. Only do this if your not using Hyper-V. This will allow Virtualbox to provide virtualization and enable the 64 bit guest options.

FAQ: I have a 64bit host, but can't install 64bit guests.

drobbins wrote:Virtualbox won't be able to provide virtualization while Windows Hyper-V service is running.

That sentence is wrong. VirtualBox does provide virtualization while Hyper-V is running. Not the one kind that you want maybe, but it most certainly does.