Using field-theoretic simulations, we study a symmetric diblock copolymer melt confined between two parallel neutral walls separated by a distance L. We consider two scenarios: a mean-field regime and a fluctuating case corresponding to a polymerization index N similar to 10(5). For both cases, we analyze the behavior of this system in the disordered and the ordered phases as a function of the film thickness L. In particular, we compute the structure factor for each case, and compare it to the predicted structure factor for a bulk system using the random phase approximation. In the disordered phase, we find qualitative agreement with the bulk system, except that the amplitudes are dependent on the modes perpendicular to the slit. The ordered structure in all cases was found to be a lamellar phase oriented perpendicular to the walls. Also, we studied the location of the order-disorder transition (ODT) in both cases. In the mean-field regime, it was found that the transition is insensitive to the width of the cavity, and occurs, as expected, at chi N* = 10.5. For the fluctuating case, we find a shift in the ODT that becomes essentially constant for slit widths larger than the unperturbed radius of gyration of the copolymers. For thicknesses below this value, we observe a confinement induced melting of the film, for which a theoretical explanation is proposed.