A new campaign by malicious hackers uses a website designed to look like Microsoft's Windows Update page to trick unwitting internet users into infecting their computers with a Trojan horse remote access program, according to antivirus experts at Sophos.

The scam uses email messages that appear to come from Microsoft to get recipients to visit a web page that uploads the malicious program.

The messages have subject lines like "Update your windows machine" or "Urgent Windows Update".

A link in the body of the email message appears to take users to the Microsoft Windows Update website, but would actually forward them to a site operated by the attackers and install a Trojan horse program called DSNX-05.

The website run by the hackers was registered to an internet service provider in Toronto, but it has since been shut down. The site looked very much like the actual Microsoft Windows Update page and displayed Microsoft's corporate logo.

One clue that something was amiss was that the URL displayed in the web browser address bar showed only the IP (Internet Protocol) address of the site, instead of the Windows Update address. Sophos does not know how many internet users may have fallen for the ruse, he adds.

Using the promise of Windows software patches to distribute malicious code isn't new. However, the latest attacks show that scammers are adopting strategies used by phishers to evade detection by gateway and desktop antivirus programs, says Graham Cluley, senior technology consultant at Sophos.

Those behind the attack may have been trying to capitalize on anticipation of Microsoft's upcoming software security patch release next Tuesday, Cluley suggests. On Thursday, the Redmond, Washington, company said it intends to put out a number of security patches for its software.

"It's such a shame that, just as we're beginning to teach people more about security updates, cybercriminals are exploiting that," Cluley laments.

Sophos points out that Microsoft does not issue security warnings in the manner used by this attack. Email users should be on guard when receiving an unsolicited email that contains an attachment or asks the reader to click a link to a web page, Cluley says.

Although the Web page used in the latest attack has been disabled, those behind the scam could post the content in a new location and restart the attack, he warns, adding, "It's hard being an average internet user. You just can't trust anyone."