Computer programs that secretly plant themselves on people's computers and then monitor users' online behavior or hijack their browsers have become a scourge, according to a new report from the Pew Internet and American Life Project.

The group said tens of millions of Americans have been struck by so-called spyware and 91 percent of Internet users have changed the way they behave online as they try to avoid it.

81 percent say they have stopped opening e-mail attachments unless they are sure these documents are safe

48 percent have stopped visiting particular Web sites

25 percent say they have stopped downloading music or video files from peer-to-peer networks

18 percent of Internet users say they have started using a different Web browser to avoid software intrusions

About 93 million Internet users -- 68 percent of them -- have had computer trouble in the past year that is consistent with problems caused by spyware and viruses, though 60 percent of those who had problems were not sure where the problem originated.

About 25 percent of Internet users have seen new programs on their computers that they did not install or new icons on their desktop that seemed to come out of nowhere. One in five Internet users have had their homepage inexplicably changed.

The report says that those who have broadband connections at home and those who range far and wide online are among those most vulnerable to spyware.

Nearly half of Internet users see spyware as a serious threat to their online security. Many believe that more should be done to alert consumers when adware is being loaded onto their computers at the same time they are installing other software.

In many cases, adware is installed after Internet users check off and consent to a user agreement, but 73 percent of Internet users admit they do not always read such agreements.

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