The thing with windows is it *requires* its user to look after it (maintenance).
The average joe with their windows pc suffers from the problems like windows slowing down over time, and crashes and whatnot. I have seen peoples computers where they have installed so much *****, yahoo toolbar, google toolbar, this toolbar that toolbar. Have 20 icons in the system tray (notification area as its now known), **** loads of programs to run at startup, 70 icons on the desktop and just basically a complete mess. And yeah, those people do suffer problems.
For many of us who know how to maintain a windows PC windows XP itself is actually very stable, my PC has never crashed, no BSOD's at all. It doesn't have any of the slowdown, basically because i maintain it well.
The difference between windows and mac os is basically mac os is fool proof. Out of the box, just like Steve says "It just works", there is no maintaining, no defragging, no bloated registry to take care of, no slowdown after 3 months.
Don't get me wrong, i am loving this fact about my macbook, i can just turn it on and use it, and its basically because its quite difficult for the user to mess things up.
The problem with windows is not windows itself, but the people that use, though i admit if windows were designed better it wouldn't have that problem