The Mac isn't taking a huge bite of the PC market share. As a matter of fact, there are sold roughly as many macs as there are sold Ps3s (yearly).
The Mac sticks to between 3-4% market share on units sold (shipped), but as macs tend to last longer, the common belief is that ~ 5% of the PCs that are being used, are macs.
The Mac is now doing better (both unit and share wise) than just 3-4 years ago, at which point it stuck to 2-3% market share. That's a major increase for Apple, but a minor loss for Windows.
As I mentioned earler, the mac sells ~ 10 million WW yearly.
So if the trend continues, mac won't be a major contestant to the PC. What some people (generally Mac enthusiasts (spelling?)) believe, is that the Mac will increase to a 20-25% market share in just 10-15 years.
The reason for this, is that the mac has a market share of over 20% among the youth in Americas (ages 16-32 I believe). They believe that at once you use a Mac, you won't go back to windows. Time will prove if this is true.
I really don't believe that, as people are far less loyal to brandname than one usually believes.
But now I'm going off topic.
No - Windows is not losing serious ground to Apple (in the PC buisiness).
Edit: Forgot the link. http://www.systemshootouts.org/mac_sales.html