I've only commuted in winter once, last year in fact, and I found that boots alone were not enough. I mostly used two socks together: a thin but tightly woven wool sock on first, then a pair of those thick hunting socks that were wool on the outside with some type of fluffy liner on the inside, and for the boots a set of insulated waterproof hunting boots. I used that configuration in as low as zero degrees and never had a problem.