  All the models of elementary particles and their interactions derived from
String Theory involve a compact six-dimensional internal space. Its volume and
shape should be fixed or stabilized, since otherwise massless scalar fields
(moduli) reflecting their deformations appear in our four-dimensional
space-time, with sizable effects on known particles and fields. We propose a
strategy towards stabilizing the compact space without fluxes of three-form
fields from closed strings. Our main motivation and goal is to proceed insofar
as possible within conventional string world-sheet theory. As we shall see,
D-branes with magnetic flux ("magnetized D-branes") and the forces between them
can be used to this end. We investigate here some necessary ingredients: open
string one-loop vacuum amplitudes between magnetized D-branes, magnetized
D-branes fixed at orbifold singularities, and potential energies among such
D-branes in the compact space that result from tree-level closed string
exchanges.
