The stealthy programs have become the weapon of choice of data thieves, spammers.
Cyber-criminals are behind a dramatic rise in stealthy programs called "trojans" that infect computers to sell rogue software, send unwanted email or steal personal data, a study has found.
In a report released in London, Microsoft said the number of trojans removed from computers around the world in the second half of 2007 rose by 300 percent from the first half.
The figure has risen so sharply because more computers are fitted with software that detects malicious programs and because criminals had come to see trojans as their "tool of choice," the report said.
"The numbers have simply exploded, it's huge," said Vinny Gullotto, general manager of the Microsoft Malware Protection Center. "There is a lot of criminal intent there."
Trojans can log keystrokes to gather passwords, send spam from private computers or harvest email addresses or personal information for criminal purposes.
The most common family of trojans last year was "Win32/Zlob," a piece of malicious software, or malware, that people unwittingly download from the Internet.
Its designers trick people into saving it by telling them they need a new piece of software to watch video online.
Once installed, it bombards people with pop-up messages and bogus flashing warnings that their computer is infected.
The messages say: "Your computer is infected! Windows has detected spyware infection. Click here to protect your computer."
The trojan then sends adverts offering to sell rogue anti-spyware on sites that could expose customers to credit card fraud. Microsoft said the problem is global and linked to organized criminal gangs.
"The majority (of trojans) come from the (United) States, China, Russia and South America," Gullotto said on the fringes of the Infosecurity Europe trade conference on Tuesday.
Microsoft said the number of computers around the world that were made safe after being infected with trojans rose from one million in the second half of 2006 to 19 million in the second half of 2007.
The report is online here.