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Ever had a "friend" who listens sympathetically to the details of your personal life, then turns around and repeats the information to everyone? Some software is becoming more and more like that gossipy pal: It nestles into your hard drive--and then reports everything it sees to its creators.

Sometimes the check-ins are benign, and many software manufacturers are up-front with what their programs disclose about you. Antivirus scanners, for instance, download updates to keep themselves working properly, and might report back on viruses your PC contracts.

Other programs are nosier. Take, for example, GoZilla, a free download manager. This tool from Radiate lets you resume an interrupted download or pause large downloads--certainly a useful application for dial-up Internet users. But GoZilla also collects user demographics, keeps an eye on where you surf, and tracks your online activity. GoZilla is very blunt about its intentions: The company will share any data it collects with anyone it wants. It gets kudos for honesty, but the company's forthrightness doesn't offer much solace.

Even when the vendor's intentions are noble, snoopy software can backfire. Last year, Verant Interactive, operators of the online game EverQuest, decided to crack down on players who were using files on their hard drive to cheat. Verant decided the only way to end the unfair play was to require that any new player to a game of EverQuest allow a Verant program to scan their entire hard drive for the cheat files. After a fierce weeklong outcry against what many considered to be the digital equivalent of a cavity search, Verant backed down. The program was never released.

Still more insidious are programs that bait you with offers of great free services and then turn around and exploit your personal information. Gator, an online wallet program that makes filling in online order forms easier, also targets advertising to you based on your Web surfing history and your geographical location.

That's fine, as long as you know what you're getting into. Unfortunately, that may not always turn out to be the case. One way that Gator convinces users to accept its ActiveX applet is through a dialog box that pops up unbidden at other Web sites. Surfers who don't read the box carefully can mistakenly assume that Gator, like Flash, is necessary for viewing the Web site. If you make this assumption and click 'Yes,' Gator will slip onto your system and start to track your online behavior via cookies and other silent means.

Ultimately, there's only one way to take complete control of the information that leaks out of your system as a result of snoopy software: Get a good firewall program like ZoneAlarm. It should at least let you know when some application decides to phone home with information about you.