  We consider two-flavor QCD in the lattice regularization with improved Wilson
fermions. In this formulation chiral symmetry is explicitly broken at order a
and hence the isovector axial currents require improvement as well as a finite
renormalization if they are to satisfy continuum Ward--Takahashi identities up
to small lattice corrections of O(a^2). Algorithmic difficulties at coarse
lattice spacings, where HMC suffers from a distorted Dirac spectrum, are
discussed. This is shown to be a cutoff effect, which disappears rapidly as the
lattice spacing is decreased. An alternative algorithm, the polynomial hybrid
Monte Carlo algorithm, is found to perform significantly better in the presence
of exceptionally small eigenvalues. Extending previously used methods both the
improvement and the renormalization of the axial current are implemented
non--perturbatively in terms of correlation functions formulated in the
framework of the Schroedinger functional. In both cases this is achieved by
enforcing continuum Ward identities at finite lattice spacing. Together, this
restores the isovector chiral symmetry to quadratic order in the lattice
spacing. With little additional effort the normalization factor of the local
vector current is also obtained.
