We study the QCD sources of systematic uncertainties in the experimental
extraction of the W cross section at hadron colliders. The uncertainties appear
in the evaluation of the detector acceptances used to convert the number of
observed events into a total production cross section. We consider the effect
of NLO corrections, as well as of the inclusion of parton showers, and evaluate
the impact of spin correlations and of PDF and scale uncertainties.Comment: 16 pages, 6 figure