Add command path to Windows in elevated Command Prompt:
set PATH=%PATH%;"C:\Program Files\Oracle\VirtualBox"
Find the UUID of your VMs with:
VBoxManage list vms
To run a VM as headless, make a Batch file containing:
VBoxManage startvm {uuid} --type headless
To run a headless VM at Windows boot, make a scheduled task that runs the batch file above. Make sure to disable the time limit on task.
To stop a headless VM, make a batch file containing:
VBoxManage controlvm {uuid} acpipowerbutton
This will shutdown the VM properly. If for some reason the VM does not respond to it you can us 'poweroff' instead. This simulates a hard shutdown.