1. An apparatus for use in an image-guided surgical or a diagnostic procedure for tracking an instrument in an operating space that includes a target site of a patient, the apparatus comprising: (a) a data-storage medium for storing scan data representing scans of the operating space including the patient target site; (b) an image capture device adapted to capture at least two stereo images of the operating space including the patient target site and such instrument during the image-guided surgical or diagnostic procedure; (c) a display device; and (d) a processor in communication with the data-storage medium, the image capture device, and the display device for: (i) registering selected scan data with the stereo images without using a fiducial structure, (ii) constructing a composite, three-dimensional rendition showing features from the selected scan data and the stereo images, and (iii) displaying the composite rendition on the display device, so as to enable a user to track the instrument as it is moved within the operating space to a selected position with respect to the patient target site.