Or Linux PCs or Apple PCs.. The sales on all PCs is down, and the articles of the last few days blaming Windows 8 are simply FUD. Windows 7 is still available for sale, and will be available to OEMs to the next few years even after retail sales stop.
The reason PC sales are down is down to two quite simple reasons -
1) Tablet buying is way up. More people are choosing tablets as their second PCs rather than PCs. Most people still have PCs in their house, but often a tablet is preferred as the entertainment machine as it's portable, people can take it to bed with them (ie, great for lazy people, and people are getting lazier every year ;)), it's relatively cheap, and it can do most of what normal people do. The thing is, most tablets do NOT run Windows - they run a dedicated smart-phone/tablet OS such as iOS or Android (or one of a few other varieties). As such, these not counted as PCs in anyone's stats.
2) PCs are much, much better than they used to be. The component manufacturer's are kinda to blame here. As Moore's Law trudges on, PCs have now become more powerful than the average user requires. You know, the ones reading email and using Facebook, not the ones rendering 3D scenes. The processing bottleneck in a new PC is generally the user's brain response time. In the early 2000s, for example, I would be buying a new PC every year, to keep the PC up with my usage needs. By 2006 or so, that was down to every couple of years, and maybe some component upgrades. Now, a new PC can last me 4-5 years with no component upgrades besides maybe adding some extra hard drive space. People are buying less PCs because they don't need to buy them as often.
And likewise speaking of components, MTBF figures are up much higher than they used to be. Hard drives no longer magically crash the days after the warranty expiration. They genuinely last years. I'm running some HDDs I've had for 8 years through 3 machines. Now, when an HDD does die, we nerds replace the HDD and restore from backup (right? Right!). Average Joes buy a new PC. Couple failures occurring less often, with tablets replacing PCs as secondary (and sometimes even primary) machines, and you understand why PC sales are down.
There's no Windows exodus. We all knew at some time we'd reach a tipping point where tablet sales (well into the hundreds of millions, crossing a billion this year, devices) would start to reduce PC sales. There's not even a PC "exodus". Just as many people use PCs as did 5 years ago. Millions more in fact. It's simply that less people have bought NEW PCs.