The profile of malicious code on the Internet is changing with porn diallers and Trojan horses becoming more serious problems. A study on the malicious code blocked last year by managed services firm MessageLabs finds the spread of Trojan horses is becoming more organised.

From recording Trojans sporadically, MessageLabs is now intercepting 40-50 Trojans at a time. These are systematic attempts to infect victim's machines, it says.

Sex diallers are another growing problem. These change the number used by dial up connections to expensive premium rate lines. Such programs, which are regularly modified by their creators and pose legal problems for AV firms. By adding detection for the programs (which don't load without user interaction) the vendors could be accused of restraining trade. One man's malware is another's useful utility.

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