A commercial atomic force microscope (AFM) has been modified to operate as a nanorheometer using only a PC-based hardware component and a custom software interface. Measurements on an agar gel model substrate reveal a viscoelastic response that is well described by a recently proposed theory for viscoelastic deformation. Both spectroscopic (harmonic response as a function of drive frequency) and impulsive (relaxation following a step drive) measurements are made. The reciprocal of the frequency corresponding to the maximum phase response is found to coincide with the step relaxation time in agreement with theory. A relaxation time of 16.6 ins is obtained for the 4% agar gel, and 9.3 ins for the 3% agar gel. Both theory and experiment give a phase lead which, although counterintuitive, has a physical explanation. Material properties measured using the modified AFM show broad agreement with data generated using a conventional parallel plate rheometer and illustrate a useful basis for the comparison of measurement techniques.