  Influence of disorder-induced Anderson localization and of electron-electron
interaction on superconductivity in two-dimensional systems is explored. We
determine the superconducting transition temperature $T_c$, the temperature
dependence of the resistivity, the phase diagram, as well as the
magnetoresistance. The analysis is based on the renormalization group (RG) for
a nonlinear sigma model. Derived RG equations are valid to the lowest order in
disorder but for arbitrary electron-electron interaction strength in
particle-hole and Cooper channels. Systems with preserved and broken
spin-rotational symmetry are considered, both with short-range and with
long-range (Coulomb) interaction. In the cases of short-range interaction, we
identify parameter regions where the superconductivity is enhanced by
localization effects. Our RG analysis indicates that the
superconductor-insulator transition is controlled by a fixed point with a
resistivity $R_c$ of the order of the quantum resistance $R_q = h/ 4e^2$. When
a transverse magnetic field is applied, we find a strong nonmonotonous
magnetoresistance for temperatures below $T_c$.
