From the Apple II in 1977 to the high-end Pentium III systems of 2000, people generally upgraded their computers at a steady rate: every two or three years.
This was usually necessary to run the latest software. During this era, people would upgrade in order to run new types of software. At first, they upgraded to run word processors and spreadsheets. Later, they upgraded to run graphical programs. And in the last major era, they upgraded to use internet applications. (Running a web browser with 8 MB of RAM is not fun.) The two usual constant factors, games and porn, were also driving innovation most of the time.
But since then, sales have flattened and upgrade cycles have lengthened. What happened?
Computers had many "killer apps" in the past. A killer app is a program or general capability so great that people are willing to buy a new computer just to have it. The first killer app for personal computers was the spreadsheet, a dream come true for accountants and business managers. Other killer apps include the word processor, desktop publishing, email, and the web browser. (Databases are great, but they've been around much longer than personal computers, and home users have never really cared about them.)
The most recent killer app was Napster, and we killed that. Its death was inevitable, but it's a shame that our last great killer app was temporary and illegal. We've had replacements, but none have been user-friendly enough for us to introduce to our parents. Napster didn't need a high-end computer, so it didn't inspire many upgrades. Instead, it gave many more people enough motivation to get a computer. If you already had a computer, the most you needed was a CD burner and a spindle of blank media. Both were already cheap and widely available.
Games have helped to motivate upgrades in the past, but they're usually not a powerful enough reason alone. There will always be the niche market willing to pay $900 for a CPU and $500 for a video card every year, but they're insignificant in the big picture. For the rest of the market, games serve a secondary role: it'd be nice to play them, but they won't justify an upgrade without any other motivating factors. Even porn, the other constant motivator, has peaked - there aren't any mainstream porn applications now that can't be served by a 5-year-old computer. Every modern computer can display pictures and videos.
Last year, laptops outsold desktops for the first time. This makes sense - portability is a killer app. With lower prices, better batteries, thinner and lighter hardware, and wireless networking, laptops are far more attractive than desktops for most users now. We've seen a boost in sales from previously desktop-only people buying laptops, but this is only temporary.
So if there's no reason for most people to upgrade, why do computers still sell at all?
Parts break. Lightning strikes. People are clumsy. People will always need to replace dead computers, but this is hardly enough to drive the industry. A working computer will generally continue to work for a very long time without destructive forces working against it. Most of the time, hardware takes a long time to die.
The industry needs something to convince people to replace computers with new models, even if all of their hardware is working and they aren't looking for any new capabilities. In other words, the industry needs methods to degrade systems enough to make them unusable:
Viruses and spyware.
Home PCs degrade to a pitiful spyware-ridden state in only a few months. When most people complain that their computer is slow, the real problem is that it's full of junk: viruses, spyware, unnecessary utilities ("Make your system faster and more stable by always running our software in the background and giving us $40 per year!"), and a stupid system tray icon for every possible piece of hardware and software in the computer.
It's a sad state. And it's even more sad that people don't know that they can fix this by reinstalling Windows. They think that computers slow down over time, and the only way to alleviate the problem is to replace them.
Most of the time, this is the only reason people buy new computers. There are only a few other forces remaining:
No wonder the computer market's sales have slumped.
Their only hopes of survival are a new killer app or more spyware. It's a good thing the spyware authors aren't slowing down, because killer apps don't happen very often, and we're in a severe dry spell.