We investigate the room temperature instability and dewetting of ultrathin (<100 nm) stable polystyrene (PS) films induced by a poor solvent, such as acetone, on a higher energy silicon substrate without an oxide layer. Instability behavior is contrasted for thermal annealing and acetone-vapor annealing and by immersion under liquid acetone. The PS films that are stable under thermal annealing are rendered unstable to dewetting when contacted by acetone, notwithstanding the stabilizing apolar van der Waals interactions in all the three systems. Relatively high molecular weight films dewet only in the liquid acetone, but low molecular weight films dewet on contact with both vapor and liquid acetone. These findings indicate the role of polar interactions engendered by acetone in the film instability modified possibly by nanophase separation aided by the substrate and nucleative mechanisms. Liquid acetone greatly enhances the instability so that the number density of holes produced in liquid acetone is about 2 orders higher than in acetone vapor. In both of these cases, the scaling (N similar to H-n) of the number of holes per unit area, N, with the film thickness, H, is found to be clearly different (n similar to 2.5-2.7) from that seen in most of the previous studies on thermal dewetting (n similar to 4). From the kinetics of hole growth, the slippage effect of PS chains can be observed in the solvent vapor-induced dewetting process but not in the dewetting under liquid acetone.