We describe the effects of disorder in a two-dimensional electron gas in the presence of a parallel magnetic field. We argue that localized states lead to the formation of local moments. The parallel magnetic field for complete spin polarization depends on the electron density and the density of local moments and we can explain the observed dependence of the saturation field of the magnetoresistance on the sample quality. The magnetic properties of the electron spins are described in terms of a Curie-like paramagnetism due to localized moments and a Pauli paramagnetism due to itinerant electrons.