  The Borexino detector measures solar neutrino fluxes via neutrino-electron
elastic scattering. Observed spectra are determined by the solar-$\nu_{e}$
survival probability $P_{ee}(E)$, and the chiral couplings of the neutrino and
electron. Some theories of physics beyond the Standard Model postulate the
existence of Non-Standard Interactions (NSI's) which modify the chiral
couplings and $P_{ee}(E)$. In this paper, we search for such NSI's, in
particular, flavor-diagonal neutral current interactions that modify the $\nu_e
e$ and $\nu_\tau e$ couplings using Borexino Phase II data. Standard Solar
Model predictions of the solar neutrino fluxes for both high- and
low-metallicity assumptions are considered. No indication of new physics is
found at the level of sensitivity of the detector and constraints on the
parameters of the NSI's are placed. In addition, with the same dataset the
value of $\sin^2\theta_W$ is obtained with a precision comparable to that
achieved in reactor antineutrino experiments.
