This guide explains how you can run virtual machines with Sun xVM VirtualBox 3.1.x on a headless Debian Lenny server. Normally you use the VirtualBox GUI to manage your virtual machines, but a server does not have a desktop environment. Fortunately, VirtualBox comes with a tool called VBoxHeadless that allows you to connect to the virtual machines over a remote desktop connection, so there's no need for the VirtualBox GUI.