No, buying software is more like joining a club with annual dues. Every year, there’s a new version, and if you don’t upgrade, you feel like a behind-the-curve loser.

There’s a time bomb ticking in that business model, however. To keep you upgrading, the software company has to pile on more features each time. Sooner or later, you wind up with a huge, sloshing, incoherent mess of a program; a pile of spaghetti code that doesn’t run well and makes nobody happy.

You’re in even worse shape if that bloatware is your operating system — the software you run all day. Just ask anyone with Windows Vista.

This year, though, Apple and Microsoft both realized that the pile-on-features model is unsustainable. Both are releasing new versions of their operating systems that are unapologetically billed as cleaned-up, slimmed-down versions of what came before.

Microsoft’s, called Windows 7, comes out in October. Apple’s, called Mac OS X 10.6 Snow Leopard, arrives on Friday, a month earlier than announced. (Apple to Microsoft: “Surprise!”)

Apple’s release strategy is highly unorthodox: “Leopard, a k a Mac OS X 10.5, was already a great OS-virus-free, nag-free and not copy-protected. So instead of adding features for their own sake, let’s just make what we’ve got smaller, faster and more refined.”

What? No new features? That’s not how the industry works! Doesn’t Apple know anything?

And then there’s the price of Snow Leopard: $30.

Have they lost their minds? Operating-system upgrades always cost a hundred-something dollars! ($30 is the price if you already have Leopard. If not, the price is $170 for a Mac Box Set that also includes two suites of Apple software: iLife (iPhoto, iMovie, iDVD, iWeb and the GarageBand music studio), and iWork (the Numbers spreadsheet, Pages word processor and Keynote presentation software).

In any case, Snow Leopard truly is an optimized version of Leopard. It starts up faster (72 seconds on a MacBook Air, versus 100 seconds in Leopard). It opens programs faster (Web browser, 3 seconds; calendar, 5 seconds; iTunes, 7 seconds), and the second time you open the same program, the time is halved.

“Optimized” doesn’t just mean faster; it also means smaller. Incredibly, Snow Leopard is only half the size of its predecessor; following the speedy installation (15 minutes), you wind up with 7 gigabytes more free space on your hard drive. That, ladies and gents, is a first.

Unfortunately, Snow Leopard runs only on Macs with Intel chips — that is, Macs sold since 2006. If you have an older Mac, you’re stuck with Leopard forever.

(Techie note: Popular conception has it that the space savings comes from removing all the code required by those earlier chips. But that’s not true, according to Apple. Yes, that code is gone, but new 64-bit code, described below, easily replaces it. No, Apple says that the savings comes from “tightening up the screws,” compressing chunks of the system software and eliminating a huge stash of printer drivers. Now the system downloads printer drivers as needed, on demand.)

As it turns out, Apple programmers could not leave well enough alone. They disobeyed the original “no new features” mantra. As they pored through all the bits of Mac OS X, they kept stopping and fixing little things that had always bugged them, or coming up with neat little ways to make things better. So:

The Mac now adjusts its own clock when you travel, just like a cellphone. The menu bar can now show the date, not just the day of the week. The menu of nearby wireless hot spots now shows the signal strength for each. When you’re running Windows on your Mac, you can now open the files on the Macintosh “side” without having to restart. Icons can now be 512 pixels (several inches) square, turning any desktop window into a light table for photos.