Not in it's current form...Gary King said:Longhorn looks somewhat promising, though. It must at least be a new threat to Mac OS X.
Honestly, if I were in charge of Microsoft (lets say starting three or four years ago) I would have started longhorn by stripping it down to the foundational levels of Windows NT... remove Internet Explorer from the system... lock up the registry so no one could write to it... and started building the operating system back up from that point. I would redesign the APIs so that software was more like the package stuff that Apple is using and didn't install too much stuff in the system itself (avoiding things like dll hell).
Microsoft had a great start with Windows NT technology (which was based on OS/2) but their implementation of it was flawed. The biggest problems Microsoft has had has been throwing new features and technologies at it's releases without thinking about the consequences or actual needs for them.
Most of the Windows exploits have been based on pointless features that almost no one used. The first major viruses on Windows came because Microsoft installed and enabled Visual Basic Scripting by default. Why? Less than 5% of Windows users knew anything about it and the ones that did would have been savvy enough to install it themselves.
Sadly, Microsoft sees a stripped down version of Windows as a form of attack on them and their position in the industry... what it really would be is a great place for them to start making a truly useful and secure operating system.
But, Microsoft is run by a corporate mentality which pushes file format lock in and providing it's own software with access to the system that other venders don't have (again, opening those products to being used for exploits).
This is something that doesn't happen in Mac OS X. Apple's own software is nice, but it doesn't take advantage of any thing that other developers don't have access to. Which means you aren't locked out of features by not using Apple software (I use OmniWeb instead of Safari, Curator instead of iPhoto, Watson instead of Sherlock, etc.).
And when Apple made APIs for TextEdit and Pages to have the ability to work with tables, these APIs were made available to all developers (in fact Stone Design used these changes to add tables to Create).
So no, Longhorn isn't much of a threat to Mac OS X.
Even though Apple also has a corporate mind set in many areas, it looks like it is run by a bunch of tree huggers by comparison to Microsoft.