The PC market continues to shrink - alarmingly so if you are one of those OEMs that rely upon PC volume to maintain a viable business. That could be a result of consumers delaying upgrades to get a Windows 10 PC, but I very much doubt it.
Much more likely is that a PC duty-cycle has been extended significantly. Better hardware and software has meant that performance doesn't drop off until many years of usage have been completed.
As an example, the Acer Ultrabook I bought in 2010 and the MacBook Air I bought in 2011 are both still perfectly serviceable machines. The latest versions of Windows 10 and OS X run comfortable on them and but for the arrival of hybrid machines in the form of Microsoft's Surface I would be happily using one or the other of them.
For business and enterprise users there are taxation benefits to both three and five year life-cycles for computer equipment, but for home users there are no reasons to replace a machine which is working well. The net result? Falling sales.
Its identical to what's been happening in the tablet market, although there it happened much more quickly - probably because there are a more restricted set of tasks that tablets are used for.
Apple is seeing growth in the marketplace because a greater proportion of people replacing their PCs are choosing a Macs, whilst Microsoft is seeing large growth from a low starting point because it has built premium devices backed by a quality user experience. I suspect that these two will continue to buck the general market trend for a little while now.
For the common or garden PC vendor though, there are more tough times ahead and competing only on price, whilst possible in the enterprise market, isn't going to fly with consumers.