Ignoring the banking issues that have dominated the economy in recent years, I largely associate the decline in PC sales with increasing copyright enforcement, DRM, etc.
In particular, I find it interesting that XP hung on so long and was the one that MS started its Geniune Advantage program with. I suspect it's the first one that many people paid for (to shut up GA) and they felt more ownership of, and commitment to, making it a harder sell for them to upgrade. For better or worse, in earlier years, Microsoft did not really prioritize copyright enforcement as much as they do today. In fact, Gates is on record over this. In 2007, Bill Gates said in Fortune Magazine "It's easier for our software to compete with Linux when there's piracy than when there's not." A year earlier he said "as long as they're going to steal it, we want them to steal ours. They'll get sort of addicted, and then we'll somehow figure out how to collect sometime in the next decade." (LA Times) I think the strong change from the lax enforcement approach in the late 90s and early 2000s (before the XP GA stuff) is a major source of Microsoft's decline in sales. Their OS sales 15 years ago, IMHO, benefited greatly from social effects and word of mouth. I'm sure I'll get some flack for that sentiment, but they deliberately worked to eliminated that effect and now people, at their request, are treating software more like a valuable asset -- and hanging onto what they bought for much longer.
It doesn't help that Vista was poorly received, nor does it help that tablets are filling a lot of the appliance use-cases (video chat, YouTube - another nightmare for copyright - and pocket gaming), and it certainly doesn't help that Windows 8 is also launching like a lead balloon. People never like change. But tablets are still benefiting from the "cool new thing" network effect that has largely left PCs over the past decade, usurped largely by Apple, proving that it hasn't gone away.
People aren't interested in having a better PC than their neighbor any more though because, due largely to copyright enforcement and DRM, they feel less welcomed/excited by the PC+Internet environment and, to top it off, they are already putting their "bling" money into tablets smartphones, etc.
The last time PC sales tanked like this was in 2001, after the IT bubble burst (interestingly, Napster was shut down in July 2001). If we ignore that as an oddball (but one that's consistent with my assertion), from the early 90s to the time of Genuine Advantage, things were lined up very well for the PC market to see strong growth: copyright law was ambiguous or unenforced, and people were actually enjoying buying PCs because they could do new/exciting things every day (music, movies, games, and sharing over the internet) in spite of the specter of copyright litigation. Furthermore, desktops and laptops (full systems, allowing even the OS to change) were the only game in town - so it was the thing being talked about around the water cooler (today it's the tablet, etc).
In particular, I find it interesting that XP hung on so long and was the one that MS started its Geniune Advantage program with. I suspect it's the first one that many people paid for (to shut up GA) and they felt more ownership of, and commitment to, making it a harder sell for them to upgrade. For better or worse, in earlier years, Microsoft did not really prioritize copyright enforcement as much as they do today. In fact, Gates is on record over this. In 2007, Bill Gates said in Fortune Magazine "It's easier for our software to compete with Linux when there's piracy than when there's not." A year earlier he said "as long as they're going to steal it, we want them to steal ours. They'll get sort of addicted, and then we'll somehow figure out how to collect sometime in the next decade." (LA Times) I think the strong change from the lax enforcement approach in the late 90s and early 2000s (before the XP GA stuff) is a major source of Microsoft's decline in sales. Their OS sales 15 years ago, IMHO, benefited greatly from social effects and word of mouth. I'm sure I'll get some flack for that sentiment, but they deliberately worked to eliminated that effect and now people, at their request, are treating software more like a valuable asset -- and hanging onto what they bought for much longer.
It doesn't help that Vista was poorly received, nor does it help that tablets are filling a lot of the appliance use-cases (video chat, YouTube - another nightmare for copyright - and pocket gaming), and it certainly doesn't help that Windows 8 is also launching like a lead balloon. People never like change. But tablets are still benefiting from the "cool new thing" network effect that has largely left PCs over the past decade, usurped largely by Apple, proving that it hasn't gone away.
People aren't interested in having a better PC than their neighbor any more though because, due largely to copyright enforcement and DRM, they feel less welcomed/excited by the PC+Internet environment and, to top it off, they are already putting their "bling" money into tablets smartphones, etc.
The last time PC sales tanked like this was in 2001, after the IT bubble burst (interestingly, Napster was shut down in July 2001). If we ignore that as an oddball (but one that's consistent with my assertion), from the early 90s to the time of Genuine Advantage, things were lined up very well for the PC market to see strong growth: copyright law was ambiguous or unenforced, and people were actually enjoying buying PCs because they could do new/exciting things every day (music, movies, games, and sharing over the internet) in spite of the specter of copyright litigation. Furthermore, desktops and laptops (full systems, allowing even the OS to change) were the only game in town - so it was the thing being talked about around the water cooler (today it's the tablet, etc).