MS's hegemony over government computers is pretty remarkable, given that the DOD walked away from Apple in '85 b/c they were a single-source vendor, and the DoD wanted to be able to buy (hardware) from multiple sources, not realizing they were hitching themselves to MS's (software) wagon for the next 25+ years.
This hegemony exists not only over gov computers, but over the vast majority of the private sector as well. Every financial sector company I have worked in (3 large banks + 2 hedge funds) uses MS. I'm not aware of a single financial sector company that doesn't use MS as standard.