The following is user-contributed documentation on how to run vboxwebsrv as a service in Windows 7 / 2008 +. I am unable to verify the validity of these instructions.
1) install VirtualBox
2) menu start --> administrative tools --> task scheduler
3) create a new task
4) put name and description. Under security option, click on "change user or group", if needed to select the user VirtualBox is supposed to run as. Than select "run whatever the user is log on or not" and check the box "hidden"
5) tab triggers --> new --> begin the task: At startup
6) tab Actions --> new --> Action : start a program
Program Script : browse to find an select vboxwebsrv (should be in C:\Program Files\Oracle\VirtualBox)
7) DO NOT ADD ARGUMENTS
8) check tabs conditions and settings for custom choices
9) Also don't forget to make a directory junction. If you don't do this, all your VMs will be inaccessible.
Directory junction has to be created using this command (replace [username] with your user profile name)
C:\Windows\System32\config\systemprofile>mklink /j .Virtualbox C:\Users\[username]\.VirtualBox
10) have fun, test rebooting your machine