Microsoft used to publish lots of PC games. Now, though, they mostly don't. Sure, there's Gears of War, and a three years late port of Halo 2, but aside from that? Not much.
And it doesn't look like there'll be much in the foreseeable future, either. The company's internal Flight Simulator team has been canned. Ensemble Studios, the team behind the Age of Empires series, were broken up and shut down. And porting a Halo game to the PC three years after it appeared on the original Xbox is some kind of sick joke.