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 Centre. “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. Once installed, a 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. “The majority of trojans come from the US, China, Russia and South America,” Gullotto said at the Infosecurity Europe trade conference.