Douglas Burrows / Getty Images In 1991, an era when nobody had to worry about the future of the PC industry, a Costco in California stocks its shelves with Packard Bell computers

Research firm IDC (which is part of my former employer, IDG) has released new numbers on PC sales, and there’s only one way to describe them: they’re uuuuuuuuuuuuuuugly.

How ugly? Worldwide shipments in the first quarter were down 13.9% over the first quarter of 2012. That’s not only worse than IDC’s already gloomy expectations — it’s the biggest drop since 1994, when the company started publishing these quarterly figures.

It’s tempting to blame the bad news on Windows 8, but it can’t shoulder all the guilt — IDC said Apple is shipping fewer Macs these days too, so it’s conventional personal computers in general that are suffering from doldrums, not ones based on a particular operating system.

Are people not buying Windows PCs and Macs because they’re spending their dough on iPads and other tablets instead? Well, maybe. It’s worth noting that in December 2009, right before the iPad was announced, IDC said it expected PC sales in 2013 to be growing by double digits. (That’s yet more evidence that trying to predict long-term sales of technology products is not an inexact science, but a hopeless one.)

I can’t help but think, though, that the first signs that the PC market might be maxing out came in early 2007, before Windows 8, the iPad or even the iPhone had any influence on the business. That’s when Microsoft released Windows Vista, and an enormous number of consumers and businesses responded by saying, essentially, “No thanks, we’re perfectly happy with Windows XP.” Even today, almost a dozen years after XP’s release, the company is trying to convince a fair chunk of the PC-using world that it didn’t perfect the PC operating system back in 2001.

It’s not just that people are stubbornly refusing to see newer versions of Windows as superior to older versions of Windows. Back in the 1990s and early years of this century, PC hardware was getting better at such a rapid clip that new PCs were often far better than the machine you’d bought two or three years earlier. Today, even a four- or five-year-old PC may still have more processing power, RAM and disk space than you need. And the industry is having trouble coming up with new features that large numbers of people find irresistible.

I’m a fan of ultrabooks myself, but if they’ve failed to restore the PC business to vibrant growth, it may be because they’re about stripping out features rather than adding new ones — they have smaller screens, less storage, fewer ports and no DVD drive. That’s a tough sell to garden-variety PC buyers who have been conditioned to expect next-generation computers to offer tangible new stuff, not tasteful minimalism.

People, generally speaking, are sensible. The PC market grows when the industry gives them computers they see as meaningful improvements on what they already have. It stays stagnant, or shrinks, when new PCs don’t look that much better.

Folks aren’t going to stop purchasing conventional computers altogether, of course; the PC industry isn’t the PDA industry. But maybe it’s turning into the TV market — one in which a typical household buys the best product it can afford, then holds on to it until it breaks or a true great leap forward (like HDTV) comes along.

There is one meaningful caveat: IDC doesn’t include Windows tablets and tablet-laptop hybrids (like notebooks with detachable keyboards) in its study, which means that it’s not accounting for sales of most of the most interesting, forward-looking Windows 8 computers. Nor does it include iPads, Android tablets and other devices people are using to perform tasks that look like personal computing to me.

If IDC and other research firms defined PC as “a computing device used for general-purpose personal and business productivity, capable of running user-installable software,” their numbers wouldn’t show the market in decline. Instead, it would look like it was booming — and we pundits would all be writing about how enthusiastically consumers and companies were adopting the radically new sorts of PCs that the industry was introducing.