  The two-loop O(g^4 mt^2/mw^2) corrections are incorporated in the theoretical
calculation of MW, sin^2 theta_MSbar(MZ), and sin^2 theta_eff, as functions of
MH. The analysis is carried out in a previously proposed MSbar formulation and
two novel on-shell resummation schemes. It is found that the inclusion of the
new effects sharply decreases the scheme and residual scale dependence of the
calculations. QCD corrections are incorporated in two different approaches.
Comparison with the world average of sin^2 theta_eff leads to MH= 127 +143 -71
GeV and MW= 80.367 +/- 0.048 GeV, with small variations among the six
calculations.
