Computers are made and marketed as all-purpose machines, but for a lot of people they have a rather limited job description: Web browsing, e-mail, MP3s, digital photos, the occasional letter ...

Sound familiar?

Those are roles that any remotely new computer can easily fill. Unfortunately, just because any random computer will suffice doesn't mean that you should buy any random computer.

Instead, find out how the computer will be used once it's plugged in. If the user isn't interested in installing new software but would rather use the tools that come with it, you should shop for a Mac.

That's not the cheapest option, though. A Mac Mini, iMac desktop or MacBook laptop — starting at $599, $999 and $1,099, respectively — will often cost more than a PC with about the same storage and processing power. But it is the easiest option.

An Apple machine will be much simpler to set up and maintain, thanks in large part to Mac OS X's outstanding record of security. It also will include Web, e-mail, photo and music software far superior to the junk on most PCs.

A Mac can read and write almost all PC files, including Microsoft Office documents. Apple's switch to Intel chips even lets a Mac impersonate a PC, running Windows with the help of such software as Apple's free Boot Camp.

Why go the Windows route, then? The best selling point for Microsoft's operating system is the unparalleled variety of software and hardware that runs on it. The diversity is especially deep in games and business-productivity applications.

If you intend to take advantage of that selection, or if the programs you use often don't have Mac equivalents, you'll do well with Windows.

Windows PCs also come in some sizes and shapes absent from Apple's lineup, such as cheap, big "desktop replacement" laptops. But most PCs do little to distinguish themselves from one another.

A test of three new desktop PCs — Dell's Dimension E521, $619; Gateway's eMachines T5048, $450 before a $50 rebate; and Hewlett-Packard's Pavilion Slimline s7600e, $965 before a $50 rebate — showed how Windows computers have become a commodity.

Aside from the HP's size — its encyclopedia-size box fit easily on a desk, unlike Dell and Gateway's bulky "tower case" enclosures — these computers differed only marginally in their hardware.

The eMachines PC, for example, shipped with a loud cooling fan and a cheap, roller-ball mouse whose innards would require cleaning every few months.

The Dell featured "DataSafe," a backup system built around a second hard drive, and a convenient keyboard with helpful shortcut buttons and two extra USB ports.

HP included a wireless-networking receiver and a "LightScribe" CD/DVD-burner that can — slowly — print a grayscale label on special blank discs.

The most important difference between the eMachines PC and the other two didn't jump out until after running a Microsoft testing utility. Like many low-end models, the T5048 had far too little memory for Windows Vista, the resource-intensive XP successor due in stores Jan. 30.

If you're worried about Vista compatibility, you can't just look for a "Vista Capable" computer. For full Vista support, you need one labeled "Premium Ready."

The software bundles on all three were the usual mix of last year's releases of basic productivity programs, inferior photo or music software (for example, the Yahoo Music Jukebox that Dell insists on bundling) and Web-security suites that will ask you to pay for a subscription in a month or so.

Tech support ought to be a tiebreaker among all this fine print, but I've yet to see anybody provide it with consistent excellence. Calls probably will be answered quickly or accurately, but not both.

With any computer, Mac or PC, keep a few hardware requirements in mind: