I like your analysis, but this statement is self-contradictory: "it's the most vulnerable, but also has the most momentum behind them". I think that Windows, Office... is the least vulnerable part of Microsoft, but it's also low-growth.
There's a format lock-in in place. Microsoft's greatest asset is the fact that it controls the file formats used by business (.doc/.xml, and the new xml based ones aren't better or more open).
This creates a de-facto requirement to buy microsoft software - in many cases you need a copy of MS Office to render or interact with a document in a way they're used to, and all the documents in a company already are in that format, so the path of least resistance is to buy another copy of MS Office.
The web avoids the issue as it embraces non-proprietary formats (or ones that are well understood). Once people get their data in a better place, that's the beginning of the end for MS.
Office is low growth with respect to percentage growth. It makes massive money now, and needs massive new revenue to grow quickly. With that said, their growth in absolute dollars is money that every other company would love to have, including Apple, Google, and Facebook.
