  We analyze the generation of helical magnetic fields during preheating in a
model of low-scale electroweak (EW) hybrid inflation. We show how the
inhomogeneities in the Higgs field, resulting from tachyonic preheating after
inflation, seed the magnetic fields in a way analogous to that predicted by
Vachaspati and Cornwall in the context of the EW symmetry breaking. At this
stage, the helical nature of the generated magnetic fields is linked to the
non-trivial winding of the Higgs-field. We analyze non-perturbatively the
evolution of these helical seeds through the highly non-linear stages of
symmetry breaking (SB) and beyond. Electroweak SB occurs via the nucleation and
growth of Higgs bubbles which squeeze the magnetic fields into string-like
structures. The W-boson charge density clusters in lumps around the magnetic
strings. After symmetry breaking, a detailed analysis of the magnetic field
Fourier spectrum shows two well differentiated components: a UV radiation tail
at a temperature T ~ 0.23 m_higgs slowly growing with time, and an IR peak
associated to the helical magnetic fields, which seems to follow inverse
cascade. The system enters a regime in which we observe that both the amplitude
(\rho_B/\rho_{EW} ~ 0.01) and the correlation length of the magnetic field grow
linearly with time. During this stage of evolution we also observe a power-law
growth in the helical susceptibility. These properties support the possibility
that our scenario could provide the seeds eventually evolving into the
microgauss fields observed today in galaxies and clusters of galaxies.
