PC sales trends aren't a proxy for how well Microsoft is doing anymore
The reason is simple: since Windows 7 the OS and MS Office run quite well on old hardware. A little less than 4 years ago I salvaged an old Latitude D420 from the office, maxed its RAM to 1.5GB and loaded Win7 32-bit on it. Loaded Office, MS Security Essentials and the normal stuff (Flash, PDF reader, etc.), created a standard account for my 8 yr-old daughter and gave it to her. Believe me, that thing needed practically zero maintenance and worked great for her entertainment and school work until the HD died about 6 months ago. I then gave her an old Acer tablet with a detachable keyboard on which I had loaded Win8 (previews and now RTM). Again, it works great for her.
I also salvaged a Latitude D430 around the same time and did the exact same thing, gave it to my (then) 12 yr-old daughter and had the same result. It's actually still running, but when the Surface RT was released I got her one. Again, she's loving the Surface... and I'm loving the peace of mind I get from its very sturdy build, curated app store and zero maintenance requests.
I also got a Surface RT for my wife, who uses it constantly and brings it with her all the time. Again, peace of mind for the same exact reasons.
Myself, I've still got my 4 (maybe 5?) yr-old Dell M4400 on Win7, but I'm seriously considering the Surface Pro when it gets released.
My point is that, in four years I found no need to purchase new hardware until the Surface turned up. In fact, I'm not upgrading any of my non-touch PCs to Win8. What for? Windows 7 is still the best non-touch OS out there, and it works very well for what I use it for. Also, I don't think anything from Apple or Linux will ever be able to take the non-touch OS market from Microsoft, and they will continue to own that market until it goes away.
As developers slowly migrate to touch-based UIs for their applications the traditional PC form factor will die out. Microsoft is already there for them, and its OS will only get better. Meanwhile, sales of Windows hardware will not go through any surges or drastic drops... simply because there is no need to buy newer, more powerful hardware to run the new OS and Office. People will wait for their old hardware to die before spending their money on new Windows PCs -- and those will most probably be of the mobile, touch-based variety. Microsoft did good by their users in giving them new operating systems that do not require new, more powerful hardware. In the case of Microsoft's hardware partners, though, that didn't help them very much.
So, ignore these "analysts" who predict Microsoft is doomed because PC hardware is not selling as much as before. Their viewpoint is myopic (if not deliberately ill-intentioned). Microsoft is transforming itself into a Services + Devices company, and if you experience their ecosystem the way me and my family does (Win8 + Nokia Win Phone + SkyDrive + Outlook.com + XBox + Live accounts, etc.), you will see just how powerful it is... and they haven't yet fully integrated Skype, XBox, Azure, etc. into the mix. 2013 will be the year Microsoft shows the world what it has built. However, the blogosphere haters still gonna hate... that is just how they are. Just ignore them... and enjoy what Microsoft is giving us: the true future of cloud-based computing.