One of the most insidious things about E.T. applications is that most computer users have no idea these invaders are in their computers. Steve Gibson, a computer consultant from Irvine, Calif., learned he had an E.T. application on his computer only when he was running Zone Alarm, an Internet­fire wall application. It listed applications on his computer that he knew were in contact with the Internet, such as his Internet Explorer browser and his Eudora e-mail. Then it asked him about one he'd never heard of: tsadbot.exe, which turned out to be an E.T. application he had unwittingly let into his computer a month earlier while downloading some shareware.

What can the average computer user do to guard against intrusions? Nothing — but Gibson wasn't an average user. Outraged, he developed a program called OptOut (available free until July 1), which removes Radiate. He's working to extend it to other E.T. applications, including Conducent's adbot, the mysterious tsadbot.exe that he found lurking in his computer.

One big question is, What will the law have to say about E.T. applications? Privacy advocates claim existing statutes ban many of them. The Computer Fraud and Abuse Act, which was enacted to prohibit hacking of government computers, contains some broad language about unauthorized access to computer data. There's also a good chance Congress may step in and pass new legislation that bans computer spying more directly. In any case, the first lawsuits have already been filed: a class action against RealNetworks seeks $500 million in damages on behalf of 1 million RealJukebox users in California.

Even before E.T. applications have their day in court, though, public resentment may be leading companies to be more cautious about using them. In an attempt to catch hackers who were crashing servers, EverQuest, a popular online role-playing game, devised an E.T. program that searched users' hard drives for hacker programs. As soon as it was announced, Verant Interactive, the company that makes EverQuest, was flooded with angry e-mail. "I got one from a veteran saying, 'I fought in Vietnam for the rights of this country, and one of those rights is the right to privacy,'" says Verant Interactive CEO John Smedley. In the face of the criticism, Smedley decided to dump the E.T. application and replace it with technology that looks for hackers on the company's servers. "It's probably not going to be as effective," he says. "But, hey, that's life."

Even Microsoft, which evoked the ire of privacy advocates with Registration Wizard, has joined the privacy crusade. It introduced a spot on Microsoft.com called Profile Center, which it says allows users to examine every piece of data Microsoft has collected about them and delete data they don't want Microsoft to have. Yusef Mehdi, Vice President of Marketing for MSN, says Profile Center "has grown from the lessons we learned from Windows 95." One of the lessons, he notes, is that a well-advertised privacy policy can make business sense. "If you do that, you will inspire much more consumer confidence," he says, "and they will give you more data."

The new sensitivity many companies are exhibiting is good news for computer users concerned about privacy. Yet for every Radiate or Verant that gets caught and cleans up its act, there are probably more that haven't been caught — and are still spying. In the long run, Cheswick says, the answer may be to segment computer hard drives physically into public and private areas so downloads don't have access to information people want to keep confidential. For now, he has a simple solution: he just doesn't download applications from the Internet or from e-mail. That may seem drastic. But if you go the other route, don't be surprised at what your computer tells the world about you.