Why does virtualbox only have 32-bit option, no 64-bit option on Windows 7?

I have downloaded and installed the newest virtualbox 4.3.20 for my Windows 7 (64-bit OS), but when I want to install 64-bit Linux, the New->Create Virtual Machine only displays 32-bit option, no 64-bit option. Could anyone give any clues about this issue? Thanks very much in advance!

Solution:

Take a look: http://www.fixedbyvonnie.com/2014/11/virtualbox-showing-32-bit-guest-versions-64-bit-host-os/

If VirtualBox is only showing 32-bit versions in the Version list make sure:

• You have an x64 CPU installed. (Optimally, a 64-bit OS should also be installed to receive acceptable virtualization performance.)
• Hardware virtualization is enabled in the BIOS. (Your CPU must support it.)
  • For Intel x64: VT-x (Intel Virtualization Technology) and VT-d are both enabled
  • For AMD x64: AMD SVM (Secure Virtual Machine) is enabled
• Hyper-V (or any other form of bare-metal hypervisor) is not installed