To start a machine headless (without the VirtualBox windows around) use
$ VBoxManage startvm centos1 –type headless
where centos1 is the name of my virtual machine. To list all the virtual machines use
$ VBoxManage list vms
This works on Windows with cmd too, but you have to move to the VirtualBox directory at first, like this
cd C:\Program Files\Oracle\VirtualBox
VBoxManage modifyhd “C:\Users\Chris\VirtualBox VMs\Windows 7\Windows 7.vdi” –resize 81920