Converting existing PC into a VM with Virtual Box
I like to use Disk2VHD for creating virtual disk. This virtual disk can then be used inside any created VirtualBox virtual machine.
This tool will also "reset" drivers on Windows so they will boot and detect their new virtual environment without any issues.
If no one else comes up with an answer / all in one tool then I would suggest simply using the VMWare converter tool and importing the virtual machine to Virtual Box.
Using this tool, you do not need to use a second machine.