  We present a semi-analytic treatment of galactic winds within high
resolution, large scale cosmological N-body simulations of a LCDM Universe. The
evolution of winds is investigated by following the expansion of supernova
driven superbubbles around the several hundred thousand galaxies that form in
an approximately spherical region of space with diameter 52 Mpc/h and mean
density close to the mean density of the Universe. We focus our attention on
the impact of winds on the diffuse intergalactic medium. Initial conditions for
mass loss at the base of winds are taken from Shu, Mo and Mao (2003). Results
are presented for the volume filling factor and the mass fraction of the IGM
affected by winds and their dependence on the model parameters is carefully
investigated. The mass loading efficiency of bubbles is a key factor to
determine the evolution of winds and their global impact on the IGM: the higher
the mass loading, the later the IGM is enriched with metals. Galaxies with 10^9
< M_* < 10^10 M_sun are responsible for most of the metals ejected into the IGM
at z=3, while galaxies with M_* < 10^9 M_sun give a non negligible contribution
only at higher redshifts, when larger galaxies have not yet assembled. We find
a higher mean IGM metallicity than Lyalpha forest observations suggest and we
argue that the discrepancy may be explained by the high temperatures of a large
fraction of the metals in winds, which may not leave detectable imprints in
absorption in the Lyalpha forest.
