1. A guide system, comprising: a data processing apparatus for generating one or more images of a subject, said images being co-registered with the subject; a display for displaying the images to the user; a probe; and a tracking unit for tracking the location of the probe and transmitting said location to the data processing apparatus, wherein, said data processing apparatus generates said co-registered images of the subject in a main display area when the probe is within a defined three-dimensional visualization region, and upon a user moving the probe to a defined three-dimensional interface region outside of the visualization region, the data processing apparatus ceases to generate the co-registered images and causes the display in the main display area of an icon representing the probe and one or more virtual buttons, each of said virtual buttons being associated with a corresponding instruction to the system, and wherein the data processing apparatus is arranged to obtain a selection by the user of any of the virtual buttons, said selection effected by positioning the physical probe to position said icon relative to the apparent position of said virtual button, and to modify the generated images based on the selection.