  We numerically investigate the transport properties of interacting spinless
electrons in disordered systems. We use an efficient method which is based on
the diagonalization of the Hamiltonian in the subspace of the many-particle
Hilbert space which is spanned by the low-energy Slater states. Low-energy
properties can be calculated with an accuracy comparable to that of exact
diagonalization but for larger system sizes. The method works well in the
entire parameter space, and it can handle long-range as well as short-range
interactions. Using this method we calculate the combined effect of disorder
and interactions on the Kubo-Greenwood conductance and on the sensitivity of
the ground state energy to a twist in the boundary conditions. We find that the
influence of the interactions on the transport properties is opposite for large
and small disorder. In the strongly localized regime (small kinetic energy,
large disorder) interactions increase the transport whereas for weak disorder
(large kinetic energy) interactions decrease the transport.
