Cyber-criminals are behind a dramatic rise in programs called “Trojans” that infect computers to sell rogue software, send unwanted email or steal personal data, a study has found.

According to Reuters, the recently released report from 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.

Microsoft says 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. 

Microsoft said the problem is global and linked to organized criminal gangs, Reuters reported.

“The majority (of Trojans) come from the (United) States, China, Russia and South America,” Gullotto said.

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, Reuters said.

The report can be accessed online at http://www.microsoft.com/security/portal/sir.aspx.