A virtualization program creates a kind of 'sealed box' which simulates a normal computer, but runs completely within the software environment of an existing operating system. Into that 'sealed box', the user can install another operating system, which then 'thinks' it's running on an ordinary PC. When the virtualized system is shut down, all the information and settings it contains is saved in a single large file on disk. There are several virtualization systems available for Ubuntu, but the best-known is VirtualBox (formerly VBox), a free program from Sun Microsystems (http://www.virtualbox.org). VirtualBox can be installed via the Add/Remove option on the Applications menu.
VirtualBox files are stored in the user's home directory in a hidden folder called .virtualbox. Apart from the basic setup information, this consists of two folders called HardDisks and Machines. The Machines folder contains the settings used by VirtualBox when starting and running a virtualized system; the HardDisks folder contains the actual information file which makes up each virtualized system.
Creating a new system image
Users creating a new virtualized system are taken through a wizard where they specify the settings. Popular systems like Windows XP have their own predefined settings which should need very little tweaking to work on the user's system. They will need to create a new 'hard disk' file which will be used to store the system; this can be either a fixed size, or a dynamic file which will be resized up to a fixed limit as files are added to the system. A 10Gb maximum should be enough for just about any operating system. Once the file is created it can be copied and moved around like any other file: so your completed Windows XP installation can be transferred via network, memory stick or hard drive to any PC running VirtualBox.