  We evaluate one-loop diagrams in the unitary gauge that contribute to
flavor-changing neutral current (FCNC) transitions involving two and four
fermions. Specifically, we deal with penguin and box diagrams arising within
the standard model (SM) and in nonrenormalizable extensions thereof with
anomalous couplings of the W boson to quarks. We show explicitly in the SM the
subtle cancelation among divergences from individual unitary-gauge
contributions to some of the physical FCNC amplitudes and derive expressions
consistent with those obtained using R_xi gauges in the literature. Some of our
results can be used more generally in certain models involving fermions and
gauge bosons which have interactions similar in form to those we consider.
