Pop quiz: When are reading a Web site's privacy policy and declining to receive spam or to share your personal information not enough to protect your privacy? Answer: When the site suddenly changes its policy--and your choices.

That kind of switcheroo has been happening a lot lately. Privacy policy changes have left legions of users of BestBuy.com, EBay, Hotmail, and Yahoo Mail struggling to sort through a thicket of options to avoid junk mail.

Yahoo Mail, for instance, replaced a single check box in which users could decline to receive e-mail ads with a series of more than a dozen radio buttons that were all set by default to include users in spam mailing lists.

MSN's free Hotmail service should receive some credit for swimming against the tide. Hotmail altered its privacy policy to give users more options for keeping their information out of marketers' hands. But if you signed up for the service before March 2002, you must revisit the site to protect your data (e-mail address, birth date, zip code, gender, and occupation). For instructions on protecting your privacy at Yahoo Mail and Hotmail, see Internet Tips.

Have you checked your EBay privacy settings lately? In February, the online auction house changed its privacy policy to reserve the right to reveal anything about you to nearly anyone. EBay requires registered users to pony up a real mailing address and telephone number, not just an e-mail address. That policy means that EBay and its partners can harass you not only through your in-box, but also by snail mail and over the telephone.

Click My eBay and the Preferences link, followed by Change My Notification Preferences, to get to the page where you can opt out of telemarketing calls and junk mail. Also, be sure to scroll down and pay particular attention to the Other Emails and Other Contacts sections.

Meanwhile, at BestBuy.com, the retailer decided in June that it would merge the customer databases of its Web site users with the purchasing history of its store customers, thereby creating a massive database that the company plans to employ for targeting advertising to each user.

Best Buy spokesperson Joy Harris justifies the company's change this way: "As a shopper myself, it makes no difference to me where [retailers] keep [customer databases] or how they merge them.

"You have to assume they have customer information about [you]," Harris says, "and who cares?"

Well, I care, Joy, as do millions of other people besieged by spam and ads on their computers. The lesson: The price of privacy is eternal vigilance. Whether you register to use a free online service or you purchase something from an online store, don't neglect to check your account's privacy settings periodically to ensure that the site hasn't suddenly "given you an opportunity to receive marketing offers."

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