When VirtualBox Doesn't Show 64-bit OS Options

VirtualBox (and VMware Player, for that matter) have a dropdown for the OS of the virtual machine. Sometimes, 64-bit options don't appear. That usually happens for one of two major reasons:

1. Hardware virtualization is not enabled, and it's required for 64-bit virtual machines. Check your BIOS and enable Intel VT-x or the AMD equivalent.
2. Another application is holding exclusive control of hardware virtualization. That other program is usually another hypervisor, but I've heard that some antivirus solutions do this. The most likely culprit for Windows hosts is Hyper-V, which runs all the time as a system service if you have it installed. Uninstall it or stop the service to use a different hypervisor.