Oracle launched another salvo in the virtual machine battle (see The battle over virtual machine software for a deeper analysis of the battle) by offering Oracle VM 3.3. As mentioned in that ...

Some users are unable to see 64-bit guest operating systems displayed on the Oracle VM VirtualBox application and because of this, they are not able to install such operating systems on VirtualBox.

VirtualBox is flexible enough to handle many types of 32-bit and 64-bit configurations. If you need to run a 32-bit version of XP in a 64-bit host, you can do that without any problems.