First, accounting software derives very little advantage from integration into the operating system. It can be easily substituted with another program. Integration into Office is another thing entirely, but so complex Microsoft is just now getting to that.
Second Money was not a very successful product and not well liked by home users. Further, Intuit aside from a near monopoly, had several unique advantages - on-line financial agreements and integration with tax preparation software.
Microsoft tried to buy Intuit to get their on-line banking agreements for bill paying and money transfers. Microsoft's stated goal was to charge a "vig" on every online financial transaction made.
Microsoft's lawyers screwed up (didn't file a response to an information request on time) and the Intuit buy was aborted.
Microsoft tried to integrate tax preparation with Money using their own tax software but discontinued that product because of low acceptance and the fact that sloppy programming and taxes don't go together. Intuit now owns outright TurboTax (for self preparers), TurboTax Pro (for small professional preparers) and Lacerte (for professional and institutional tax preparers).
This does not mean Microsoft is not preparing to take Intuit's business. They are currently preparing an all-out assault on QuickBooks and will leverage .NET, Longhorn and Project Green (total integration of accounting with Office), so you can expect Intuit to soon be as endangered as Adobe and Google (but they'll have to solve that tax preparation problem).
As for Adobe Acrobat - that's obviously extremely endangered because similar capabilities can be integrated into Office along with Digital Rights Management, making it less convenient and less "integrated" to use pdf.
Google is also an easy target since Microsoft controls Windows, Internet Explorer and MSN. Third party search functions can easily be made less convenient to access, and that has proved sufficient to drive out competition for other products.
So, in conclusion, Quicken is a much more difficult target, but it is a target. It'll be a while before Microsoft can really crush it, but Adobe Acrobat and Google are targets that are immediate and accessable.