As I mentioned in a previous article, I consider the key to Microsoft's monopoly its proprietary formats. Without complete control over the most commonly used document formats like .doc, .xls, and .ppt, the average desktop user would no longer be locked into Windows. Users could choose an operating system that's stable, fast and powerful (such as Linux), or even one that has lots of pretty icons, shiny silvery icons (Apple OS).
Finally there's a move to create an ISO standard for commonly used documents. Oasis has begun the process of defining open XML document standards that even Microsoft will eventually have to support.