<i>PG: Would you consider funding a startup aiming to take on a massive but completely monopolized market, e.g. desktop OSes?</i><p>From what I've seen I'd say that YC has a high probability of funding the sort of brave/foolish people who would try such a thing.  Keep in mind that to a first approximation, YC funds <i>teams</i>, not <i>startups</i> -- PG has stated many times that one of the most important things a startup can do is be ready to throw everything out and start moving in a completely new direction.