The laptop computer business is doing better than ever, but the idea of the laptop computer is getting scrambled in the process.

Instead of a lightweight computer that's meant to be used on the go, many laptops these days weigh too much to be toted any farther than from a desk to a sofa. Weirder yet, some manufacturers act as if consumers don't even care what a laptop weighs.

HP doesn't make a single consumer laptop lighter than 6.5 pounds, and its print ads fail to list the poundage of any of its portable models -- even those in the thin-and-light category. At Toshiba's Web site, you must download a spec-sheet document, thick with fine print, to see what kind of dent each machine will put in your shoulder.

The underlying trend here is that so many laptops are bought to take the place of desktop machines. As the cost and performance gap between desk-bound and portable machines has steadily shrunk, consumers have started going with the computers that take up less space. (The scarcity of desktop systems that aren't built around bulky tower cases also may be a factor.)

Computers bought along these lines are best thought of as all-in-one desktop systems with built-in uninterruptible power supplies. They won't rack up any travel miles, nor are they even likely to spend much time on laps -- 10 pounds of warm computer quickly becomes uncomfortable in that position.

Some vendors, however, seem to be taking the popularity of this "desktop replacement" concept a little too far -- as an excuse to give up on portability entirely.

They appear content to forgo even minimal attempts to trim the weight of their products, shipping eight-, nine- or 10-pound behemoths (even while some competitors offer lighter machines at the same price and with the same capabilities). It's hard to escape the feeling that some folks in the industry have just gotten lazy.

Many customers, in turn, have obliged this practice by continuing to buy machines on clock speed alone, even though the slowest new chip is more than fast enough for most uses.

This is a big mistake all around. The history of computing is one of unexpected uses -- consider, for instance, how the MP3 format, an obscurity seven years ago, has made the PC many people's stereo system.

Today, WiFi wireless networking is having a similar effect, inviting customers to pick up that laptop and move it around the house -- and outside, thanks to the growing number of cheap or free WiFi hot spots at coffee shops, bookstores and libraries.

So why buy the laptop that weighs more if you don't have to? That's why my laptop shopping list starts with this concern. If you need to take the computer multiple places in a day, anything much over five pounds will be intolerable. Even if you don't think you'll be moving the computer, seven pounds seems a realistic limit.

Above that, an all-in-one desktop from vendors such as Gateway and Sony (and Apple, whenever it resumes selling the iMac) will probably offer a more comfortable keyboard and display at a lower price.

Screen size comes next. Because laptop displays are viewed at much closer distances than desktop monitors, you can comfortably use a screen smaller than a desktop's. Fourteen- or 15-inch LCDs should be fine for most folks; their widescreen variants are well-suited for DVD playback and multitasking. Seventeen-inch displays generally add too much cost or weight.

Remember to look up a screen's resolution in pixels, or picture elements. Because different sizes of screen can share the same resolution, bigger isn't always better. For example, Apple's 14-inch iBook G4 has the same pixel count as the cheaper 12-inch model.

Next, consider the laptop's memory. Get at least 256 megabytes of random-access memory, and preferably 512. Get at least a 40-gigabyte hard drive. Get at least a combination CD-RW/DVD-ROM drive, and if you opt for a DVD burner, DVD+RW is the easier format to work with.

Some computers helpfully include a fourth storage option: slots for the memory cards used in digital cameras and handheld organizers.

A laptop without WiFi is like a bike without wheels. Given the minimal difference between the 802.11b and 802.11g flavors of this standard, you might as well get the faster "g" option. An Ethernet port and a v.92 modem remain necessary too.

A laptop's processor should be next on the shopping list -- not because its relative speed is likely to matter, but because of what it can do for battery life. If you have a choice between a Pentium 4 and a Pentium M processor, go with the latter, which uses less electricity (its clock speeds understate its performance anyway).

In terms of expandability, the rule is simple: The more USB ports, the better. A FireWire port is a good idea if you own an iPod, edit home movies or plan on doing either. Parallel and serial ports are most likely useless if your peripherals date from the current century.

Bluetooth -- a wireless equivalent to USB -- is still hard to find on consumer laptops, and unless your cell phone supports Bluetooth you're unlikely to get much use out of this technology.

Expect the bundled software to be mediocre, especially on most Windows machines -- I'm still waiting to see a vendor (besides Apple) bundle a modern Web browser that can block pop-ups. Plan on downloading better programs to replace what's on board, but first switch on a firewall and grab any available security updates.

Tech support is also a frequent weakness and often comes at a price after the first few months or a year. Just in case, keep a tech-savvy friend's phone number handy.

Living with technology, or trying to? E-mail Rob Pegoraro at rob@twp.com.