  Searching for extrasolar planets around stars of different metallicity may
provide strong constraints to the models of planet formation and evolution. In
this paper we present the overall results of a HARPS (a high-precision
spectrograph mostly dedicated to deriving precise radial velocities) program to
search for planets orbiting a sample of 104 metal-poor stars (selected [Fe/H]
below -0.5). Radial velocity time series of each star are presented and
searched for signals using several statistical diagnostics. Stars with detected
signals are presented, including 3 attributed to the presence of previously
announced giant planets orbiting the stars HD171028, HD181720, and HD190984.
Several binary stars and at least one case of a coherent signal caused by
activity-related phenomena are presented. One very promising new, possible
giant planet orbiting the star HD107094 is discussed, and the results are
analyzed in light of the metallicity-giant planet correlation. We conclude that
the frequency of giant planets orbiting metal-poor stars may be higher than
previously thought, probably reflecting the higher precision of the HARPS
survey. In the metallicity domain of our sample, we also find evidence that the
frequency of planets is a steeply rising function of the stellar metal content,
as found for higher metallicity stars.
