Numerous systems correspond to confined magnetic particles interacting through dipolar interactions: magnetic particles, flux lines in superconductors, charged colloids. From careful extended Monte-Carlo simulations the ground state is obtained and shown to be derived from a triangular lattice with a few layers stuck to the confinement boundary. The most common solid state obtained by relaxation from a random configuration is found to be an excited state composed of many triangular crystallites separated by lines of alternate topological defects with five and seven nearest neighbours. The melting of these two solid phases gives a common liquid phase. But the melting of the ground state is a sharp first order transition while the melting of the excited state is a continuous vitreous transition which starts at low temperature with the motion of topological defects on their defect lines, while the rest of the sample remains solid, as a two component medium. The elastic properties of these solid states under shear are also considered. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.