Commonly known as spyware, the software can hijack or even paralyse a computer.
It can cover a user's screen with pop-up adverts, and can tracks a user's movements on the internet. It can log every key stroke to send personal information, including credit card numbers, back to the spyware's authors.
One user, Shelley Emerson, a 34-year-old single mother, had used the internet to keep in touch with far-flung family members and friends. Now she complains her PC is almost useless under the current barrage of spyware programs.
"It was so easy to stay in touch with everyone, to give and get information, to send them pictures of my twins," she says.
"Now my computer has become so full of spyware I can hardly use it."
Software giant Microsoft and computer makers Dell agree that the spread of spyware has turned the internet into a dangerous jungle.
Computer help columns in newspapers are filled with requests from users whose machines have been rendered unusable.
"I've got this really nasty spyware, 'about:blank', that has invaded my computer and won't let me use the internet very much and keeps taking over my home page," complained one reader in Chicago.
"I must have downloaded something I wasn't supposed to. Ordinarily I'm very suspicious about opening websites, but somehow they got me."
"People are pulling their hair out and many have no idea that spyware even exists"
computer repair expert
The expert's 30-line reply was ominous: "You can fix this," he wrote. "But it won't be easy."
Neil Bettencourt, who runs a computer repair business in California's tech-savvy Silicon Valley said about 50% of his calls are from people whose computer has been hijacked by spyware.
"People are pulling their hair out and many have no idea that spyware even exists," Bettencourt said. "They just cruise around the internet with no protection."
According to the National Cyber Security Alliance, spyware lurks on at least 80% of the computers in the US. Dell says spyware is the single largest complaint of people calling its tech support lines, and The Los Angeles Times reported that even Bill Gates recently found his computer infected by the malicious software.
Worse than spam
For many victims, spyware can be worse than spam, which fills inboxes with objectionable junk but rarely paralyses a computer.
"Spyware is turning out to have a lot of similarities to spam -not technically, but because of the fact that it has gone from being annoying to sapping productivity, and now it is turning out to be a security concern," says International Data Corporation (IDC) analyst Brian Burke, who said that two out of every five tech support calls nationwide are spyware related.
Spyware lurks on at least 80%
of the computers in the US
Some unscrupulous companies market tools that purport to knock out the software but that actually install it on host computers.
IDC pegged anti-spyware sales at $47 million last year but expects that to more than double this year to $106 million.
The intensity of the threat has spurred the usually slow-moving Microsoft to take action. The software giant realises that if spyware is not checked, it will present a serious obstacle to the high tech Holy Grail of having every gadget and device linked over the internet.
The company recently bought the small anti-spyware firm Giant, and in early January rolled out the test version of a spyware fix that will be available for free later this year. Yahoo! also offers a free anti-spyware tool that can be downloaded by users, as does AOL.
"Spyware makers will continue to write new programs that circumvent existing protections"
However, these programmes do not solve the problem. They just give computer users a tool to clean their computers once they become infected.
"They are OK for tech-savvy surfers, but for casual internet users, who know nothing about computers, spyware will continue to be an enormous problem," says security analyst Steph Marr.
"Spyware makers will continue to write new programmes that circumvent existing protections. To keep your computer clean you will need to update and scan your computer frequently. Most people just won't do it."