  We generalize the spectral-curve construction of moduli spaces of instantons
on $\MT{4}$ and $K_3$ to noncommutative geometry. We argue that the
spectral-curves should be constructed inside a twisted $\MT{4}$ or $K_3$ that
is an elliptic fibration without a section. We demonstrate this explicitly for
$T^4$ and to first order in the noncommutativity, for $K_3$. Physically, moduli
spaces of noncommutative instantons appear as moduli spaces of theories with
$\SUSY{4}$ supersymmetry in 2+1D. The spectral curves are related to
Seiberg-Witten curves of theories with $\SUSY{2}$ in 3+1D. In particular, we
argue that the moduli space of instantons of $U(q)$ Yang-Mills theories on a
noncommutative $K_3$ is equivalent to the Coulomb branch of certain 2+1D
theories with ${\cal N} = 4$ supersymmetry. The theories are obtained by
compactifying the heterotic little-string theory on $T^3$ with global twists.
This extends a previous result for noncommutative instantons on $\MT{4}$. We
also briefly discuss the instanton equation on generic curved spaces.
