NEW YORK — Internet users worried about spyware and adware are shunning specific Web sites, avoiding file-sharing networks, even switching browsers.

Many have also stopped opening e-mail attachments without first making sure they are safe, the Pew Internet and American Life Project said in a study issued Wednesday.

"People are scaling back on some Internet activities," said Susannah Fox, the study's main author. "People are feeling less adventurous, less free to do whatever they want to do online."

Like no other Internet threat before it, spyware is getting people's attention, she said. "It maybe will bring more awareness of all kinds of security issues."

Linda Parra, a technology usability consultant at an insurance firm in Madison, Wis., is typical of the once-burned, now-vigilant crowd.

Hit twice by spyware, after which all her Internet searches went to a rogue search engine rather than Google, she bought the safer Mac computer, installed two layers of firewalls and began switching off her broadband-connected machine when she's out.

"I've become a lot more security conscious," she said, adding that she had to learn much more about how computers and the Internet work.

Parra also banned her daughters, ages 12 and 14, from game sites.

"All it takes is one click ... and you can end up going somewhere you don't want to go and getting a little bonus pack (spyware) with your freebie," she said. "I believe that's what happened."

Spyware generally refers to unwanted programs that often sneak onto computers without their owners' full knowledge. A subset called adware covers software designed to display targeted ads to subsidize another program's development.

While some computer users knowingly install spyware and adware, they often hitch rides with games, screensavers and other freebies, or exploit security flaws in Microsoft Corp.'s Windows operating systems and Internet Explorer browsers.

According to Pew, 48 percent of adult Internet users in the United States have stopped visiting specific Web sites that they fear might be harboring unwanted programs.

Twenty-five percent stopped using file-sharing software, which often comes bundled with adware.

Rogue programs can also disguise themselves as songs or movie files awaiting download on file-sharing networks.

Eighteen percent of U.S. adult Internet users have started using Mozilla Firefox or another alternative to Internet Explorer.

In addition, 81 percent have become more cautious about e-mail attachments, a common way for spreading viruses, though rare for spyware or adware.

All told, 91 percent have made at least one behavioral change.

Users hit by spyware or adware were more likely than others to change their habits.

Avi Naider, president of adware company WhenU.com Inc., said he's not surprised.

Although in theory, adware is about exchanging value for value — free software for ads — in practice, some in the industry engage in deceptive practices and alienate consumers such that they "just stop visiting Web sites," Naider said.

Although many users have changed their online habits, they haven't necessarily fixed their machines, even as infected computers slow, often to a crawl.

Twenty percent of users who had computer problems did not attempt a fix. Among those who did, 29 percent waited a month or longer.

Two in five who tried to fix their machines did so on their own while others needed help from a friend, family member or a professional repair shop. In 20 percent of cases, the problem couldn't be fixed.

The survey also found that 43 percent of Internet users say they've been hit with spyware, adware or both. Those who report spyware were more likely to have previously engaged in "risky" behavior such as playing online games and visiting adult sites. Broadband users tend to be at greater risk.

Pew also found that three-quarters of Internet users do not always read user agreements and other disclaimers where spyware and adware are sometimes disclosed. The study was based on random telephone-based interviews with 2,001 adult Americans conducted May 4 to June 7. It has a margin of sampling error of plus or minus 2 percentage points.