1. A guide system for use by a user who performs an operation in a defined three-dimensional region, the system including a data processing apparatus for generating images of the subject of the operation in co-registration with the subject, a display for displaying the images to the user, a probe having a position which is visible to the user, and a tracking unit for tracking the location of the probe by the system and transmitting that location to the data processing apparatus, the data processing apparatus being arranged, upon the user moving the probe to a selection region outside and surrounding the defined region, to generate one or more virtual buttons, each of the buttons being associated with a corresponding instruction to the system, the data processing apparatus being arranged to register a selection by the user of any of the virtual buttons, the selection including positioning of the probe in relation to the apparent position of that virtual button, and to modify the computer-generated image based on the selection.