Certified UNIX?
If one wants a certified, secure, stable operating system that runs on a system without an Apple badge, you could do a lot worse than trying Solaris 10 x86. It's used by millions of corporates who demand performance, stability and security (many customers run big iron on Solaris, including most of the world's banks, financial institutions and data centres). Customers that do the Sun thing would giggle at the idea of running on Windows - a typical Solaris system is so stable that it has an uptime measured in years, not hours or days. Windows (and I dare say, even Linux or OS X) can't hold a candle to that. I've certainly never heard of a Windows/Linux/OS X system capable of having CPUs and memory added/removed to/from it, without requiring an OS restart. In the commercial Solaris world, this kind of feat is commonplace with the bigger iron.
Sun has also open-sourced Solaris (and they have had a positive history of tracking down bugs in the code), which has only served to make the OS even more secure than it was before. You can get your favourite UI on Solaris - if you don't like the Sun Java Desktop, you can quite easily get Gnome or KDE and run it just as you would on a Linux system. Did I also mention that Solaris x86 is free to download and use (with free security updates, but paid support for everything else)? Sun makes its money out of charging for support, but as a home user you can quite easily get away with no support package - since security updates come free, and if you really fancy the features in the next release, you can always download it for free.
As for Apple? I consider them a non-option for most of the stuff I would buy a computer for (not to mention that the low regard Steve Jobs holds his customers in, gives me no desire to join that group of unfortunates. Besides, I perceive Mac users as a bit gay, for the most part: They're always on about being Mac users ... just as gays are always on about how they're homosexual. Anyone else ever notice this? That being said, I personally know four Mac users who act perfectly normal.) Getting back to cost, the cheapest Apple that is expandable in the same way as a PC is the Mac Pro, which is hideously overpriced compared to the cheapest expandable PC that will meet my needs. The rest of the non-notebook Mac range are toys, quite frankly. Until Apple realises that they're competing with $500 PCs that are expandable (and that most computer users still expect a system to be expandable), the Mac will remain a niche platform - like the Commodore Amiga once was. And being a niche platform is quite dangerous - you only need to be bettered once by the mainstream market to be wiped out completely by the economies of scale: Just ask Commodore. They were once much bigger than Apple, and now they're history.
Security on Macs can best be described as "unknown". Since the OS is not open-source, holes cannot be fixed so quickly by the security-researching masses. (Apple also seems to be spending more of their time and resources on the iPhone than OS X these days, a concept which is just laughable for any computer manufacturer). On the other hand, I can't think of one sensible black-hat hacker actually revealing an exploit for the Mac to the masses (they're far more likely hanging on to such exploits for their own illegal uses), so those actually blind enough to claim that "proof" is required (in the form of a documented hack) are being disingenious at best (or at the very least, think that proving a negative is a trivial matter). Hacking these days is, after all, largely for profit: If you wanted to rob a bank, and you found a secret back door, would you tell the world about it before you had a chance to use it? Would you tell anyone even after you used it? Didn't think so...
Getting back on subject, Vista is something I will avoid at all costs. My next laptop WILL have XP Pro on it - any OEM salesman that thinks he can foist Vista on me will be given an ultimatum: Supply XP, or lose the sale. (Any hardware manufacturers reading this, take note! Microsoft isn't the only one capable of playing hardball). If all else fails, I will be buying a three-pack of OEM licences for Windows XP Pro come January 2008, just to make sure I do not ever have to install Vista on any forseeable computer I build or buy. Probably still cheaper than buying one copy of Vista Ultimate, too.
Microsoft can do one thing to get themselves back into my good graces: Drop what they're doing on this useless Vista SP1, and get XP SP3 out pronto. The idea of having to wait 3 years for a service pack update is, quite frankly, ludicrous. Besides, it appears that nobody with any sense is buying Vista anyway - so their first priority should be to support paying customers.
Oliver.