A coalition of major Internet companies today will announce a plan to weed out advertisers who infect computers with unwanted code that spies on users' activities or generates nuisance advertising.

The five companies -- America Online Inc., Yahoo Inc., Cnet Networks Inc., Verizon Communications Inc. and Computer Associates International Inc. -- said they are responding to escalating frustration among consumers about stealth computer code that can generate endless pop-up ads, monitor which Web sites people view or even capture personal data and impair computer performance.

The code, known as spyware, adware or trackware, is generally delivered to people's computers via e-mail, through programs that consumers download or even simply by visiting a Web site. Often, there is no easy way to remove it.

Although no firm data exists on how much spyware and adware is distributed, AOL estimated after a survey last year that some form of spyware or adware is on 80 percent of the nation's personal computers. Another survey by the Pew Internet and American Life Project estimated that 91 percent of U.S. computer users have made at least one change in their online behavior to avoid spyware or adware.

The coalition's plan focuses on consumer downloads of programs such as games, file-sharing services, screen savers or other applications for everything from improving computer performance to displaying local weather data.

Such programs often contain spyware or adware from advertisers or marketers who pay the providers of the software to be included in the product being downloaded.

Under the plan, which will undergo a test phase until next spring, providers of downloadable programs would have to ensure that there is prominent notification that spyware or adware is included and explain what the code does. There must be easy means of deleting the spyware or adware, and the origin of the advertising must be clearly displayed.

Downloadable applications that meet the criteria would then be placed on a "white list" of certified programs that are safe to download. The participating Internet companies will not distribute, or advertise on, programs not on the list.

The system will be overseen by Truste, a nonprofit company that provides security and privacy seals of approval to Web sites that adhere to its guidelines.

The system was developed in conjunction with the Federal Trade Commission and the Center for Democracy and Technology, a digital advocacy group.

FTC Commissioner Jon Leibowitz said he has spoken with advertisers who would prefer not to use adware distributors but fear that if they stop, their competitors will gain too much advantage.

With this program, he said, "it will be harder for those [adware] companies to continue their reprehensible practices."

Fran Maier, executive director of Truste, said that the white list, and the Internet firms' commitment to abide by it, will be more powerful than a simple safety-seal program.

Two major Internet companies, Microsoft Corp. and Google Inc., are not participating, though they said they consulted on the effort and support its goals.

"We continue to review the beta program and will re-evaluate our participation once the beta cycle has been completed," according to a statement from Microsoft.

A Google spokesman said that "we think there are positive aspects but would have preferred that some parts of it be stronger."