You are not required to re-partition your physical hard drive in order to install VirtualBox on your Windows system. VirtualBox will acting as a virtual computer on your physical computer. The virtual hard drive is build on a physical hard drive within the Windows.
Virtual computer and its components (such as hard drive) are acting as files on Windows system only.
For testing purpose, you are required to give about 10-20GB to the virtual hard drive for the virtual BackTrack.