Ads are a fact of life in every medium, including the Internet. Most people don't mind them, most of the time, and in some cases they're a source of information and entertainment.

People who buy computer, or fashion, magazines do so as much for the ads as for the articles, and newspaper ads for car dealers, movie theatres and supermarkets are pored over by readers. Some television commercials, such as those that debut during the Super Bowl or the Olympic Games, are seen as entertaining in their own right.

But some kinds of ads just drive people nuts. And right at the top of that list are the so-called pop-up ads spawned by Web sites, or by America Online. These ads, which open in their own windows, can dim the joy of Web surfing, or even of using your computer, by cluttering up your desktop. Some refuse to close in the normal fashion, or even spawn additional ad-laden windows when you try to dismiss them.

There are technically two types of pop-up ads. The classic kind, which appear in front of whatever material you're reading, seem to be most popular on AOL, though they still exist on the Web. The second type is called a "pop-under." While it doesn't cover the active window, it does appear in the Windows taskbar, and it does suck up some of your computer's "system resources." If you're using Windows 95, Windows 98 or Windows ME, this profusion of hidden windows you never chose to view can actually slow down or destabilize your PC.

But there are ways to block most pop-up ads. Here's how.

AOL

America Online calls its subscribers "members," but it seems to view them more as sales prospects than as anything else. Long before pop-ups were a plague on the Web, they were a curse on AOL. The service does, in fact, give its members a way to turn off pop-ups, but it's buried in a submenu and confusing.

In AOL 6 or 7 for Windows or version 5 or OS X for Macintosh, first get to the Preferences screen, from either the Settings menu or the toolbar. Then select "Marketing." Next, click on the button labelled "pop-up," and ignore the propaganda screen that tries to convince you to keep viewing them. Choose the "No" option, and click your way out of the sequence of screens. Some of the language describing your choices is less than lucid, so read everything carefully.

Two notes on this process. You must go through it separately for each one of the screen names on your account. And you have to redo this selection annually, or else AOL will revert to showing you pop-up ads. Why? Because, apparently, nothing matters more to AOL than selling you things, not even respect for your preferences.

Pop-Up Stopper

Unlike AOL, the Web has no central preferences area, no corporate control. So to stop pop-up ads on the Web, you have to install software on your PC that intercepts and kills the ads. One good free utility is Panicware's Pop-Up Stopper. It sits in the Windows system tray and, in my experience, succeeds in killing most pop-up ads.

There are actually three versions of Pop-Up Stopper. Two of them, which sell for $40 and $20, include support and lifetime upgrades, and other features unrelated to the blocking of pop-up ads. But the free version gets the job done.

One problem with Pop-Up Stopper, and other programs in this category, is that they also block some pop-up windows you want to see, such as side windows containing information on some Web sites. Pop-Up Stopper gets around this by allowing a pop-up window to open if you hold down a key when clicking on a link.

AdSubtract

This is a well-made $30 program that not only blocks pop-ups, but can strip regular ads from Web pages, to speed up surfing on slow connections. It also can block annoying animations and sound effects on Web sites, manage cookies and block "referrers," which are messages passed from one Web site to another that disclose which sites you've visited.

AdSubtract allows you to customize its features by Web site so that ads aren't blocked on certain sites where you'd like to see them. There's a free version, but while it blocks regular banner ads, it doesn't affect pop-ups. You can download both versions at www.adsubtract.com.

Others

There are myriad other utilities (some free, some not) on the Web that claim they can squash pop-ups. If you visit a shareware-downloading site such as www.download.com and type in the word "pop-up," you'll get a list of programs with names like Popup Free, PopUpBuster, Kill Popup and PopupCop. Unlike the two programs listed above, I haven't tested these. But if you don't like one, you can try another.

Also, the Macintosh version of Norton's Internet Security package ($100), besides antivirus and firewall protection, includes a program called Privacy Control that blocks both banner and pop-up ads. (The Windows version blocks banner ads only.)

There's one more thing you can do to stop pop-up ads if you hate them: boycott the companies who use them. I do. In my mind, a company whose marketing tactics annoy me or get in my way doesn't deserve my business. I will never buy anything from the companies whose pop-up ads I encounter most frequently. These include X10, whose ads for tiny video cameras often feature the cameras scanning women, or Orbitz, an online travel service.

You can also complain to the Web sites from which you notice the pop-up ads spawning. Many of these are respectable and are desperately agreeing to any kind of ad tactic that will bring in revenue. But if their users rebel, they may have to reconsider.