The presence of an Option #2...
MS had it so good in the Wintel years (1994-2004) because most of the software made for Windows wasn't interoperable, and the open source alternatives in the mid 1990's weren't comparable at all. (Sawtooth was more disgusting to look at than Windows 95, and I hated KDE.)
Now, approaching the 20th year since Windows 95 was released, and look at what we have: Apple's alternatives are equal or better than Microsoft Office 2003/2007 in a lot of areas, Linux has gotten plain gorgeous as well with Gnome/KDE (I looove KDE 4, it is a great improvement over it's prior versions!), Google's Chrome OS is the first proof-of-concept of a Cloud-centric operating system.
While Windows 7 has caught up to Apple in terms of UI appearance and fixed a lot of problems with Windows Vista, it's not inherently different since Windows XP... or 2000, or Me, or 98, or 95. All of the above operating systems (except Goog, of course) have done a lot of work on user experience, whereas Windows' best UI improvement in the last 6 years is Windows Media Center.
Applications-wise, the only reason why to buy Windows 7 is the same reason to buy X-Box 360... most games still require Windows to run well (well, there's Wine/Cedega/vmware, but let's face it, it would be easier if game developers/graphics card manufacturers wrote for multiple platforms to begin with.) If you don't play graphics card intensive games, there is nothing tying you into Microsoft except that either you can't afford a Mac (PC still whips Apple on pricing, but even on cheap hardware, Linux is there to fill the gap), or you're afraid to leave Windows.
It's easy to evangelize open source, but it won't change Microsoft's bread and butter... their stance is exclusivity: Most of their hardware is certified to work on their machines with sparing support on other platforms, Apps only work on Windows by mutualistic license agreements and developers who write Windows-only code, and Microsoft doesn't play nice with Linux or Apple, period. (Try networking a Mac Mini and a Windows 7 PC together... you'll need to spend five or more hours, plus a couple of bottles of Rogaine to replace all that hair you ripped out... And even if you figure out how, if you post it online there will be 100's who won't be able to follow what you've done for some reason or another.) While it would be possible for them to monetize Bing and use it for more Google-esque pursuits, it's not likely to happen.