  We present a detailed calculation of the contributions of charginos, scalar
quarks, and charged Higgs boson to the $K^+\rightarrow \pi^+\nu\overline\nu$\
and $K^0_L\rightarrow\mu^+\mu^-$\ decays. We include mixings: that of charginos
and that of the scalar partners of the left and right handed top quark. We find
that the box contribution to the amplitudes is much smaller than the penguin
contribution, which can be $\sim$10\% of the Standard Model contribution, even
for relatively large SUSY masses. The charged Higgs contribution can be as
large as 25\% of the SM contribution in the first decay and as much as 40\% of
the SM contribution in the second decay.
