  We report a study of viscosity by the method of Helfand moment in systems
with periodic boundary conditions. We propose a new definition of Helfand
moment which takes into account the minimum image convention used in molecular
dynamics with periodic boundary conditions. Our Helfand-moment method is
equivalent to the method based on the Green-Kubo formula and is not affected by
ambiguities due to the periodic boundary conditions. Moreover, in hard-ball
systems, our method is equivalent to the one developed by B. J. Alder, D. M.
Gass, and T. E. Wainwright [{\it J. Chem. Phys.} {\bf 53}, 3813 (1970)]. We
apply and verify our method in a fluid composed of $N\geq 2$ hard disks in
elastic collisions. We show that the viscosity coefficients already take values
in good agreement with Enskog's theory for N=2 hard disks in a hexagonal
geometry.
