There is a file in <PATH>/Machines named, for example, Win7.vbox.
You should not edit it manually, that's done with the VirtualBox and virtual machine controls:
** DO NOT EDIT THIS FILE.
** If you make changes to this file while any VirtualBox related application
** is running, your changes will be overwritten later, without taking effect.
** Use VBoxManage or the VirtualBox Manager GUI to make changes.
<PATH> would be, in my case, /var/lib/virtual/Machines; if your virtual machine(s) is/are in your home directory, I believe it's ~/.virtual/Machines (don't remember, it might be ~/.virtualbox or something similar).
Hope this helps some.