A new Trojan doing the rounds is aimed at Skype users by pretending to be the newest release of the company's Net telephone software.

The worm is contained in an e-mail and claims to contain version 1.4 of Skype's VoIP software client, released on 10 October. It is instead a variant of the IRCbot Trojan horse, according to MessageLabs.

An ever-growing amount of unsolicited e-mail, or spam, contains malicious software, as criminal groups seek to infect computers with malware in order to add new systems to their botnets, or zombie networks, used illegally to distribute spam, said Alexander Peters, an anti-virus researcher at MessageLabs. "These groups are always looking for machines to keep their networks alive," he said. "It's a huge problem, and spam, which today is almost inseparable from malware, is at the heart of it."

Users who launch the attachment see a fake "installation error" box on their screens, according to MessageLabs. What is actually happening, however, is that the malware is installing itself, altering the registry and shutting down shared access and Microsoft's Windows update services. It then tries to connect to an IRC server but fails.

Although the Trojan is not widespread, it is still circulating, according to Peters. "What has drawn attention to this particular Trojan - and not the many other Trojans out there in cyberspace - is the fact that this malware is part of a well orchestrated phishing attack based on a very convincing e-mail that uses the name of a widely familiar company and a timely occasion: the release of a new software version," Peters said. "Users should never open attachments from untrusted senders."

Skype is concerned about the incident. The company said in a statement that it doesn't use e-mail to inform users the latest version of its software and uses digital signatures to ensure that its software releases are valid.