  The calculation of the $O(N_f\alpha^2)$ corrections to muon decay is
described. These are the 2-loop diagrams containing a massless fermion loop and
they form an important gauge-invariant subclass. It is shown that all such
diagrams can be expressed in terms of a universal master integral. We focus on
the calculation of box diagrams and in particular on the removal of their
infrared divergences.
