Someone may be eavesdropping on you -- not your phone calls or conversations, but more like a sneak attack to check up on your Web browsing.

It's estimated that millions of Americans are being watched by advertisers while on the Internet, thanks to software that you may not even know you've downloaded.

Lillian and Christine Mallia found something they didn't know was on their computer. It's the culprit behind all the pop-up ads they've been seeing. Not only are pop-up ads everywhere, but their homepage mysteriously changes.

"I logged on, and it was something totally different," said Lillian Mallia.

The culprit is called spyware or adware. It may be included in music or programs that are downloaded. It changes computer settings, adding links to advertisers' sites. And it quietly shares every move you make on the Web with advertisers.

Debbie St. Clair runs Unwantedlinks.com -- a site devoted to fighting spyware. She says there are over 900 different programs computer owners might download with spyware included.

"These adware components have the capability to monitor a computer user's every key stroke command," St. Clair said. "We're talking over 50 million people that might be infected."

Often, the spyware displays a licensing statement that many people ignore. Some will collect which web pages you view, what software is on your computer, and make a note of your first name and your zip code.

"Some of these things can be picked up just viewing a Web site," St. Clair said.

Coralee Whitcomb is with Computer Professionals for Social Responsibility, and she calls this style of advertising "sleezeware". But is it illegal?

"In fact, 'sleezeware' does not break any laws," Whitcomb said.

Ben Isaacson heads the Association for Interactive Marketing. He says the industry is working to regulate itself, including better warnings when spyware is about to be downloaded.

"Making that information more legible and more understandable is very important," Isaacson said.

Four of the top companies who make spyware products were contacted for this report, but they all either didn't return calls or declined to be interviewed.

So who's at risk of infection? Computer experts say that people who download music and shareware are very likely to contract spyware along the way, and those who do a lot of instant messaging.

And if it seems your computer has slowed down lately, look for spyware consuming your machine's resources.