Read MS's 10k and come back and seriously tell me they are an ad company. They don't have much ad infrastructure (that's outsourced), and they make very little of their money from ads. Microsoft is a software company, and some of that software is paid for with ads. Google is an advertising company.

Incidentally, this sort of proves the point that Google's ownership of platforms where ads are displayed puts them at an advantage compared to competitors, who have to go to people like MS for space.