My 2 cents:
I’ve used PCs my entire life, and know their benefits. There are more PC applications, they have a much larger market share, the best games are for them, the hardware is cheaper, etc. But in the time I have managed PCs and Macs in the same office (the last year and a half), I’ve had to re-install Windows about 15 times and Mac OS X never (we have 3 PCs, 3 Macs, and 1 Linux server in the main NACHI’s IT office).
It used to be true that Macs were better than PCs for graphic design and some other design/art applications. It’s not so much the case anymore, as Photoshop, Illustrator, etc run just as well on PCs as they do on Macs. The only place where Macs shine is video editing, where Final Cut Studio beats the competition hands down.
On the virus front–it’s true that there aren’t many Mac viruses, but that’s just because of market share.
On the market share front–it’s true that Windows has a much larger market share, and that’s due to one blunder/good decision made years ago. While Microsoft decided to let anyone build the hardware for Windows PCs, Apple was very sure they wanted to be the only ones who manufactured Apple computers. In the end this killed them on market share, but meant that their company continues to sell computers at a profit (where the Dells and HPs are just barely making a profit on their machines, selling a comodity, Apple is still selling a luxury item, and making money doing it). Also, Apple is starting to take over the notebook sector, selling more and more laptops while Dell sells less and less. Who knows where this goes over the next year or two, but Apples looking more and more like a contender nowadays. Windows isn’t necessarily winning because people like it (ask 10 people if they like Windows and tell me how many people actually like it as an OS), but there are a lot of factors that keep it on top.
That said, I think they both have their strengths. From my experience, I would say Macs are much more stable, and though they have less software, they software they do have tends to be consistent, easy to use, well thought out, and feature-rich. No, you can’t build your own Mac from scratch (without a lot of hacking) but it means that everything works together.
On the other side, PCs may be less stable and more prone to issues/viruses/malware/etc, but there’s a lot more software written for them. The software tends to be lower quality, but if you’re willing to look around and pay for good software, you can get pretty much anything done on a PC. If you’re a gamer, a PC is the way to go. If you like to constantly fiddle with your computer, a PC is the way to go. If you do certain types of CAD, a PC is often the way to go (I use SolidWorks which is an industrial design-oriented CAD application that only runs on Window).
For me, I got sick of tinkering with my computer all the time. I got sick of things constantly breaking because the new software I installed had conflicts with other software I have installed. I got sick of things always feeling like they could be done just a little bit better… so now I’m seriously considering a Mac. But it’s certainly not for everyone.
On the up-side, even if I decide to switch back, the new Macs run Windows better than a Dell 
