Reason #1: Vista runs more software
Mac OS X is a very pretty operating system, but beauty runs only skin deep. An operating system by itself is a poor thing -- it's the applications that run on top of it that matter. And here's where Mac OS X falls short. It can't run much common software, including enterprise applications and games.
Virtualization software like Parallels Desktop for Mac won't solve the problem. Parallels can't run even the most basic Vista games such as FreeCell, Hearts, Pinball, Solitaire and Minesweeper, because it doesn't support DirectX 9. And virtualization software creates big problems for enterprises with regard to volume licensing, technical support, creating standard enterprisewide images and so on. So it's no good for running enterprise applications either. Vista beats OS X here, hands down.
Reason #2: Vista is safer
As I've written before, Vista is a more secure operating system than Mac OS X. Mac OS X was easier to crack in a recent hacking contest. And security researcher Dino Dai Zovi had this to say about Vista versus Mac OS X when it comes to security:
I have found the code quality, at least in terms of security, to be much better overall in Vista than Mac OS X 10.4. It is obvious from observing affected components in security patches that Microsoft's Security Development Lifecycle (SDL) has resulted in fewer vulnerabilities in newly-written code.
Reason #3: It's the money, stupid
If you're got a lot of extra spare cash laying around that you don't care about, then by all means, buy a Mac. But if, like most of the world, you've got to work for a living, then you'll find that a Vista machine is far less expensive than a Mac.
Reason #4: The Mac is closed; Vista is open
Any person or company who wants to build a PC with Vista on it, and either sell it, or use it themselves, is perfectly free to do so, as long as they buy the operating system. Try to do that with a Mac, and you'll be spending some face time with Apple's attack dog lawyers.
Reason #5: Two words -- Steve Jobs
There's no doubt that Steve Jobs is a master marketer. And he also knows good design. But he's also vindictive, sends his lawyers after people who report leaked information about Apple products, and has hypnotized Apple users into drinking the true-believer Kool Aid. Don't reward him by making him richer.
By the way, if you're interested in a head-to-head death match about whether Vista, XP, Mac OS X, or Linux is the superior operating system, check out OS Smackdown: Linux vs. Mac OS X vs. Windows Vista vs. Windows XP. As you might imagine, I argue for Vista.