Microsoft designed Internet Explorer to let programmers add extra features through add-ons. By installing an add-on program -- toolbars, stock tickers, and program launchers, for instance -- users could wring a little more work out of Internet Explorer. Similarly, many sites use ActiveX -- a fancy word for little programs that add animation and other flashy tricks to a Web site.

Unfortunately, dastardly programmers began creating add-ons and ActiveX programs that harm users. Some of these can spy on your activities, bombard your screen with additional ads, redirect your home page to another site, or make your modem dial long-distance numbers to porn sites. Worst yet, some renegade add-ons can install themselves as soon as you visit a Web site -- without asking your permission.

For instance, some sites send a barrage of pop-up windows to Internet Explorer. As you struggle to close them all, you may accidentally click an OK button, allowing the site to install its evil add-on.

Windows XP's Service Pack 2 packs some new guns to combat these troublemakers. First, if a site tries to sneak a program onto your computer, Internet Explorer quickly blocks it, sending a warning across the top of Internet Explorer's screen. Clicking the warning reveals your options, which are basically to allow the download or software install, or to get more information about it.