Microsoft has penetrated the business sector through and through. Even if the business uses Unix servers and thin client's - somewhere in there, there's guarenteed to be an NT server or workstation. Well - the end user. The best way to make money is to take it off someone else. Broadband gets cheaper.
Users start looking at the things they can do on broadband.
Chances are they are already using a Microsoft operating system. If they haven't even got a PC yet - there is a high to VERY high chance the PC will have a Microsoft operating system on it. Users discover the multimedia rich world of the internet, and download apps and plugin's to make the experience richer. User gets media player, real player, internet explorer, quicktime etc. User gets interested in having a web page - user buys Microsoft Office or Frontpage.