I will answer your questions, you can find information on companies using Microsoft products and their cases at Microsoft's Case Studies
yeah it's a Microsoft 's page, but you can find tons of information, now you'll get a case from me.
I currently work at BP , we merged with several big companies around the world in the last 5 years (Castrol, Mobil, Arco, Amoco, Aral, TNK Russia and several more), those companies had their own applications based on Unix, Lotus, Linux, etc. It was a major movement, but currently we've 95% of all employees and contractors moved to Exchange Server, 100% of all users using Windows Servers and Windows Desktop OS, 100% of all user are with Office System 2003, 85% of all databases were moved to SQL Server 2000, still working on some Oracle databases and so on, why we did this?, the answer is, TCO cheaper, Help Desk Support faster and more reliable, Development is faster, we don't need to give specialized training to users.
We still have and will have Unix Systems, but this is because the specialized applications we have and they only exist for Unix.
BP is one of the companies that I can say 85 to 90% of the platform is Microsoft.