I prefer a Mac... because it actually works. I was a PC user for years and had to learn a hell of a lot about XP to be able to fix anything that would go wrong with it, which was a LOT.
However then I bought a macbook, and it's been the best thing i have ever owned. It's so easy to use, it just feels natural. Everything is well thought out and just makes sense. It never goes wrong, and Macs don't get viruses.
Also when i saw that Vista and Windows 7 had a completely different interface, the thought of having to learn everything all over again made me shudder.
I also have a PC still for playing a few games. But i still run XP :D