It seems like the winner in the Mac vs. PC debate differs
depending on what year it is. One gets ahead and comes out as the
better value, then the other one gets ahead a couple years
later.
In 2005, it's obvious that the Mac has the edge over Windows.
The lack of viruses and spyware, the competitive prices, and the
overall quality of the operating system make Mac OS X the
clear winner right now.
Let's talk about four years ago. Both Apple and Microsoft were
launching new operating systems: Mac OS X 10.0 and Windows XP.
When anything completely "new" is released, it's pretty much
expected that few things will work with it. And the more different
from the previous version it is, the more compatibility problems it
will have.
This was a big issue with OS X. When I first started running it,
there were no OS X applications aside from the ones Apple provided
with the OS - and maybe one or two written by independent
developers. There wasn't even an OS X version of iTunes. That
finally came along, as did Microsoft Office, and later
Photoshop.
Prior to that, if you wanted to run these applications, you had
to run them in Classic mode, which was generally slower than
running OS 9 by itself - and if all your applications were
Classic ones, why not just stick with OS 9?
Since OS X was completely new, Apple decided it would start with
essentially a blank slate, leaving out a lot of the features of the
old Mac OS (labels for files and folders, for example).
Until 10.3 came out last year, OS X was not much more than a
"software toy" - and people recognized that.
Windows XP was almost a seamless transition from Windows 2000
Professional (which is what I had running on my PC), and the
upgrade from Windows 98 or Me was relatively painless as well. You
installed the new OS and went about business as usual. Perhaps a
few games wouldn't work, but most applications didn't have a
problem.
With OS X, when you installed the new OS, you had a new desktop
altogether, a completely new Finder, and your documents weren't in
the new locations - you were essentially installing a second
operating system on your Mac, not upgrading your old one.
This made a lot of people wary, and people who had at one time
considered switching to the Mac were put off by the new operating
system - it was so different from the old one and had almost no
software available. They were promised an easy upgrade to Windows
XP on their existing PCs - and that's what many people ended up
doing.
The other issue was software companies. At one point, AOL in the
UK decided that they weren't going to support AOL on OS X at
all (they reversed that decision not too long after OS X
became the default OS on new Macs). Adobe took forever in
developing OS X versions of some of their applications, and
audio support when Mac OS X first shipped was pretty
lousy.
All that made many people think they could have the next
generation operating system without giving up software
compatibility today - if only they bought Windows XP (or a computer
with Windows XP on it).
Four years later, their Windows XP computers are slowed down to
a crawl by viruses and spyware, and the operating system - which
was only okay to begin with (and in some ways was a step back from
2000 Pro even) - is now completely outdated. Longhorn, the next
Windows version, was supposed to ship this year, and now they're
saying it won't be out until 2006!
Add to that the fact that Mac OS X has been through three major
revisions and is now a fully featured, current operating system
with a full complement of software available.
I've had no reason to switch on my PC for the past month, and
when I finally did in order to visit one website (an Apple II emulator that
ironically requires Windows), I had so much trouble with my
antivirus software trying to update itself every 3 seconds that I
gave up and plugged in my Mac again.
When Longhorn finally shows up (probably about the same time
OS X 10.5 does), things may change, but right now there's no
question which operating system is ahead. 
