Putting a stop to pop-up ads need not involve leaving the browser you're used to -- or coughing up a registration fee for the privilege. We tested four add-ons to Microsoft's Internet Explorer that remedy that market-leading program's one glaring omission: the lack of any way to regulate Web advertising. All are free to try, and all but one are free to use.

WDprojects' Internet Organizer Pro works -- but it may work you, too. Despite a setup-wizard interface that should have configured everything properly during this program's installation routine, we had to engage in a flurry of configuration-menu clicks to get Internet Organizer Pro to start killing pop-ups. Some less-than-clear English instructions do not help. Still, once wrestled into working order, Internet Organizer Pro does the job. It not only halts ad pop-ups but also disables start-page hijacking, unsolicited favorites-menu additions and selected cookies set by some Web sites. (Win 95 or newer/Win NT 4 or newer, www.wdprojects.be/organizer)

For many non-technical users, Panicware's clean, effective Pop-Up Stopper Free will be a better fit, if not the best. This basic program runs in your system tray, where a right-click menu offers access to a simple list of preferences. When Pop-Up Stopper does get into action, you have a wide array of choices on how to be notified -- one of two sound files, a flashing tray icon, a special cursor icon, a pop-up (!) note or, perhaps best, no notification at all. A helpful self-updating option functioned as promised, fetching some update patches and an explanation of each.

But Panicware's attempt to up-sell us on a companion program hit the wrong notes. A "Personal Privacy Report" option shows what Web-use data (cookies, visited addresses, search items and so on) are recorded on your system. But free utilities or moderate tinkering in Windows will erase all that information at zero cost. (Win 98 or newer/Win 2000 or newer, www.panicware.com/product_psfree.html)

Zero Popup 6.1, from Tooto Technologies, comes with a fee, but it's worth that $25 in our assessment. It successfully combines both versatility and unobtrusiveness; its system-tray icon shows up only when Internet Explorer is active. A right-click menu there lets you quickly get at its array of Web-browsing options. Beyond pop-ups, it also can squelch banner ads, "Messenger Spam" (ads sent through a Windows system-notification protocol) and start-page hijacking.

Zero Popup can also blank IE's memory of your Web use, from auto-completed forms to its cache and cookie files, each time you close out of that browser. It can even block Web-transmitted ActiveX spyware. Shortcut keyboard commands can turn off pop-up blocking on demand (along with its system-tray icon and sound notifications), or you can just hold the Ctrl or Shift keys down while clicking a link. You can also create a "whitelist" of Web sites that are allowed to spawn pop-up windows. This was our favorite in this group. ($25 shareware, Win 95 or newer/Win 2000 or newer, www.tooto.com)

If money is an object, however, then WebWasher Classic is our pick. WebWasher's browser add-on (a paid version is sold to corporations) can filter out pop-ups, banner ads, user-tracking "Web bug" graphics, cookies, types of files (for instance, embedded music) and interactive scripts. You can even bar the door to images and other objects according to how many pixels they take up on your screen, choose how much of an animation to view and select whether to replace blocked content with the image of your choosing.

Finally, WebWasher can stop sites from changing the size and position of your browser window or modifying its address or status bars. But finding all these options requires some mousing around in the program's main window. A Mac OS 8 (but not Mac OS X) version is also available but was not tested in this roundup. (Win 95 or newer/Win NT 4 or newer, www.webwasher.com)