  The DAMA/NaI Collaboration has observed a 4-sigma C.L. model independent
effect investigating the annual modulation signature in the counting rate of an
NaI(Tl) set-up (total exposure of 57986 kg day) and the implications of this
effect have been studied under different model-dependent assumptions. In this
paper we extend one of the previous analyses, the case of a WIMP with a purely
spin-independent coupling, by discussing in detail the implications on the
results of the uncertainties on the dark matter galactic velocity distribution.
We study in a systematic way possible departures from the isothermal sphere
model, which is the parameterization usually adopted to describe the halo. We
specifically consider modifications arising from various matter density
profiles, effects due to anisotropies of the velocity dispersion tensor and
rotation of the galactic halo. The hypothesis of WIMP annual modulation,
already favoured in the previous analysis using an isothermal sphere, is
confirmed in all the investigated scenarios, and the effects of the different
halo models on the determination of the allowed maximum-likelihood region in
the WIMP mass and WIMP-nucleon cross-section are derived and discussed.
