  Using the formalism of {\it renormalized} coordinates and \textit{dressed}
states introduced in previous publications, we perform a nonperturbative study
of the time evolution of a superposition of two states, the ground state and
the first excited level of a harmonic oscillator, the system being confined in
a perfectly reflecting cavity of radius $R$. For $R\to\infty$, we find
dissipation with dominance of the interference terms of the density matrix, in
both weak- and strong-coupling regimes. For small values of $R$ all elements of
the density matrix present an oscillatory behavior as times goes on and the
system is not dissipative. In both cases, we obtain improved theoretical
results with respect to those coming from perturbation theory.
