  We explore the implications of the Borexino experiment's real time
measurements of the lowest energy part of the neutrino spectrum from the
primary pp fusion process up to 0.420 MeV through the 7^Be decay at 0.862 MeV
to the pep reaction at 1.44 MeV. We exploit the fact that at such low energies,
the large mixing angle solution to the Mikheyev-Smirnov-Wolfenstein matter
effects in the sun are small for 7^Be and pep and negligible for pp.
Consequently, the neutrinos produced in the sun change their flavor almost
entirely through vacuum oscillations during propagation from the sun's surface
and through possible nonstandard interactions acting at the solar source and
Borexino detector. We combine the different NSI effects at source and detector
in a single framework and use the current Borexino data to bound NSI
non-universal and flavor- changing parameters at energies below the reach of
reactor neutrino experiments. We also study the implication of the current data
for the weak- mixing angle at this "low-energy frontier" data from the Borexino
experiment, where it is expected to be slightly larger than its value at the Z
mass. We find $\sin^2(\theta)w=0.224+-0.016$, the lowest energy-scale estimate
to date. Looking to the future, we use projected sensitivities to solar
neutrinos in next generation dedicated solar experiments and direct dark matter
detection experiments and find a potential factor five improvement in
determination of the weak-mixing angle and up to an order of magnitude
improvement in probing the NSI parameters space.
