I rebuilt a windows machine and wanted a Virtual Box virtual machine using Ubuntu. Guest additions was already installed and the Ubuntu was getting setup nicely. I was able to simply move the VM’s from one machine to the other with just a USB hard drive.
Copy the VM folders from VirtualBox VMs folder in the user’s home directory. If your windows machine is named bob then the path would be c:\Users\bob\VirtualBox VMs.
Install Virtualbox on the new machine. Start virtualbox and select add from the top menu. Navigate to the folder with your desired vm. Click through the folders till you get the .vhd file, and that’s it.