The difference is there is practically no lock-in in advertising systems--Microsoft has demonstrated repeatedly that it will use it's OS monopoly to unfairly leverage things online. If you think they have failed miserably, consider that it was only because of the anti-trust rulings that beige box makers got the right to change the start page.

Also, much of Google's "monopoly" comes from a source that is heavily protected by law: the pagerank patent. Patents are government granted monopolies with limited time; as such, there is little or no legal recourse as any mildly abusive behaviour is expected, tolerated, and what the system was designed to provide.