Fears over spyware have made more than 90% of American net users change their online behaviour, says a study.
Spyware can clog PCs making them slow and unresponsive
The dangers are making people swap browsers, avoid file-sharing programs and some websites says the Pew Internet and American Life Project research.
The well publicised risk of losing personal data, having a browser hijacked or suffering endless pop-up ads is driving the change, said Pew.
Surveys show that more than 80% of PCs are infected with unwanted spyware.
Spyware is the name given to programs that surreptiously make their way on to Windows PCs and then subject users to pop-up adverts, hijack their start page, install bookmarks or gather information about browsing habits.
The most malicious spyware programs lurk unseen on PCs and steal confidential information such as passwords or login details.
Spyware and adware often travels alongside file-sharing programs or media files that people download from the web.
Some websites also take advantage of bugs in Microsoft's Internet Explorer browser to install spyware on the machines of people that visit.
Some people have swapped browsers to avoid problems
The dangers of spyware are making people wary of what they do online like no other net threat, said Susannah Fox, lead author on the Pew Internet study.
"People are feeling less adventurous, less free to do whatever they want to do online," she said.
The research found that 91% of people have made at least one change to the way they act online because of fears about spyware, though only 43% said they had fallen victim to the programs.
Broadband users and those who visit pornographic sites or play games online were more likely to fall victim, the research showed.
The dangers were making people scale back on some net activities, said Ms Fox.
The Pew research found that 48% of those questioned have stopped visiting some websites for fear that they harbour spyware. File-sharing software has been abandoned by 25% of people because it often becomes bundled with adware.
It also revealed that 18% of net users have started using an alternative web browser such as Firefox to avoid the security problems associated with Microsoft's Internet Explorer.
Microsoft recently released anti-spyware software that helps people find and remove the malicious programs.
Anti-spyware firm Webroot estimates that 88% of PCs harbour spyware programs and most machines are home to many different strains of the software.
The Pew research found that though many people have changed online behaviour, 20% had not cleaned up their infected computer.
Free anti-spyware programs such as AdAware and Spybot are used by many people to clean up their PCs.