We have modelled the effects of bulk, outer-boundary, and interfacial disorder on conductivity and giant magnetoresistance (GMR) in Co10
10/ trilayer taking into account its realistic electronic band structure. Calculations were performed using our model [E. Yu. Tsymbal and D.G. Pettifor, Phys. Rev. B 54 (1996) 15314] extended to the systems with two-dimensional periodicity and layer-dependent disorder. The model is based on the Kubo-Greenwood formula and spin-independent disorder in the on-site atomic energy levels, reflecting the scattering by defects, within an spd tight-binding approximation. Exploring the contributions to conductivity from different layers, we find that the influence on GMR of the boundary and interfacial scattering is similar to the bulk scattering, because the conductivity is non-local and the thicknesses of films are comparable to the electronic mean free path. Increasing the spin-independent disorder causes a decrease of GMR in the spin-valve for both interfacial, outer-boundary and bulk mechanisms of scattering. We have also investigated the effect of outer-boundary and interfacial paramagnetic Co layers on GMR in the trilayer. We find that the GMR, in this case, is strongly reduced due to the strong spin-independent scattering at the paramagnetic layers and hybridization of d states of the paramagnetic layers with the sp bands.