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Is there a way to tell which of the cookies on my PC are helping, and which ones are spying?

Harlan Davis, Warrenville, Illinois

A cookie is merely a small file that a Web site puts on your PC to identify you, or to store information about you or your computer, such as your IP address. The good ones save you the trouble of logging on to the site on return visits, a big help if you use subscription news services such as that of the New York Times.

The bad cookies are placed by ad companies that pay for the right to place advertising on the sites' pages (some sites also leave their own ad cookies). These files track your visits to pages that display their clients' ads (or their own), and they may tailor the ads you see to your browsing history.

Over time, cookies can reveal your browsing habits, though standard ad cookies, like those that DoubleClick uses, can't attach a name to a specific surfing trail.

That means, if you wipe out your cookies as soon as you close your browser, the ad networks never get a chance to track your surfing from session to session.

Internet Explorer 6 and 7 and Firefox 1.x and 2 have good cookie-handling procedures. IE lets you keep first-party cookies (left by the site you're visiting) but block those from third parties: Select Tools, Internet Options, Privacy, Advanced. In the Advanced Privacy Settings dialog box, check Override automatic cookie handling (see FIGURE 1). Under 'First-party Cookies', select Accept; under 'Third-party Cookies', choose Block. Ignore the session cookies option. Click OK twice.

Firefox 2 can accept, and regularly wipe away, any cookie you haven't explicitly told it to keep: Click Tools, Options, Privacy, check Accept cookies from sites, and in the 'Keep until' drop-down box, select I close Firefox. To keep cookies from a few trusted sites, click Exceptions, and in the dialog box, enter the URLs of the sites whose first-party cookies you wish to keep in the 'Address of web site' field. Click Allow for each, and when you're done, click Close And OK. If you're using Firefox 1.5, click Tools, Options, Privacy, Cookies, check Allow sites to set cookies, and choose for the originating site only.

Many security programs, including Norton Internet Security, PC-Cillin, and Ad-Aware, also identify and destroy known spying cookies.