They make their users into part of the sales force.
For example some new versions of word seem to break backwards compatibility. For example Word 2007 by default saves the documents in a docx file. To the naive end user that means that if a friend or business partner starts sending these files then they must upgrade to the new version. Microsoft has the cheek to call the GPL viral!
It is amazing the range of development tools that microsoft put out to make developers life easier – at a price.
The new workflow (WF), presentation (WPF) and communication (WCF) stacks are very compelling for the developer. The catch is that you need to either write your apps as a web application or upgrade your entire workforce to XP or above. There are plenty of companies that have a mixed W2k and XP environment – these new tools cannot be used without an upgrade!