  We study Nekrasov's instanton partition function of four-dimensional N=2
gauge theories in the presence of surface operators. This can be computed order
by order in the instanton expansion by using results available in the
mathematical literature. Focusing in the case of SU(2) quiver gauge theories,
we find that the results agree with a modified version of the conformal blocks
of affine SL(2) Lie algebra. These conformal blocks provide, in the critical
limit, the eigenfunctions of the corresponding quantized Hitchin Hamiltonians.
