Computer-assisted system for guiding a surgical/diagnostic instrument in the body of a patient arranged on a supporting structure comprising: - a first patient marker device configured to be arranged in contact with the patient so as to be integral with a body region of a patient to be treated by means of said surgical/diagnostic instrument and including first marker elements; - a second instrument marker device configured to be coupled to said surgical/diagnostic instrument and including second marker elements; - an optical location sensor configured to locate said first and second marker elements (M1,M2); and - a processing unit connected to the optical location sensor and adapted to perform a virtual navigation based on an imported/reconstructed three-dimensional image and on the image detected by the optical location sensor for viewing on a viewing unit; - a three-dimensional representation of the body region generated based on said three-dimensional image repositioned relative to the optical location sensor reference system; and - a three-dimensional representation of at least one operating portion of the surgical/diagnostic instrument, graphically superimposed on the three-dimensional representation of the body region of the patient using the position of the second marker elements and of a model of the surgical instrument, the optical location sensor comprising: - a stereoscopic viewing system in which a first infrared ray video camera (42-a) associated with a first infrared ray illuminator (43-a) is carried by a first end (40-a) of an elongated support element and a second infrared ray video camera (42-b) associated with a second infrared ray illuminator (43-b) is carried by a second end (40-b) of the elongated support element, - the video cameras (42-a and 42-b) have respective axes (47-a and 47-b) tilted towards each other and converging in an area of view of the optimal location sensor; - characterised in that the optical location sensor comprises a motorized pivot device having a first end (45-a) integral with the elongated support element and a second end (45-b) that can be fastened to a support structure; said motorized pivot device having at least two degrees of freedom and being configured to produce the rotation of the elongated support element relative to the support structure about a first horizontal tilt axis and according to a second horizontal pan axis perpendicular to the first horizontal tilt axis; and in that the optical location sensor comprises a microprocessor control unit configured to drive the actuators that produce the rotations about the first horizontal tilt axis and the second horizontal pan axis to move the area of view in space; the microprocessor control unit being configured to move the area of view and maintain the image of the marker instrument detected by the video camera (42-a/42-b) substantially at the centre of the same image also in the case of shifting of the instrument in order to perform a tracking function of the instrument marker by the optical location sensor. - a first patient marker device configured to be arranged in contact with the patient so as to be integral with a body region of a patient to be treated by means of said surgical/diagnostic instrument and including first marker elements; - a second instrument marker device configured to be coupled to said surgical/diagnostic instrument and including second marker elements; - an optical location sensor configured to locate said first and second marker elements (M1,M2); and - a processing unit connected to the optical location sensor and adapted to perform a virtual navigation based on an imported/reconstructed three-dimensional image and on the image detected by the optical location sensor for viewing on a viewing unit; - a three-dimensional representation of the body region generated based on said three-dimensional image repositioned relative to the optical location sensor reference system; and - a three-dimensional representation of at least one operating portion of the surgical/diagnostic instrument, graphically superimposed on the three-dimensional representation of the body region of the patient using the position of the second marker elements and of a model of the surgical instrument, the optical location sensor comprising: - a stereoscopic viewing system in which a first infrared ray video camera (42-a) associated with a first infrared ray illuminator (43-a) is carried by a first end (40-a) of an elongated support element and a second infrared ray video camera (42-b) associated with a second infrared ray illuminator (43-b) is carried by a second end (40-b) of the elongated support element, - the video cameras (42-a and 42-b) have respective axes (47-a and 47-b) tilted towards each other and converging in an area of view of the optimal location sensor; - characterised in that the optical location sensor comprises a motorized pivot device having a first end (45-a) integral with the elongated support element and a second end (45-b) that can be fastened to a support structure;