An e-mail campaign designed to lure people to a bogus Microsoft Web site is making the rounds as part of an attempt to install a Trojan horse, antivirus company Sophos said Friday.

Attackers are sending out fake e-mails that claim to come from Microsoft's Windows Update. People who click on the link in the message are steered to a site that looks like Microsoft's security update site, where they are urged to download fake patches.

But should unsuspecting users download the bogus patches, they will infect their computers with the Troj/DSNX-05 Trojan horse, according to Sophos. That, in turn, will let the attackers remotely take control of the infected PC.

"Microsoft does not issue security warnings this way," said Graham Cluley, Sophos senior technology consultant. "They don't send updates in an HTML format, so don't follow the links in an e-mail. If you want to see if an update is real, you need to go to the real Microsoft Web site and check there."

People, however, are likely to click on the phony Microsoft update notices, given that they are making the rounds at the same time as Microsoft is poised to issue its regular monthly security update

Microsoft has posted a notice on its site saying that on Tuesday it will issue some critical patches for Windows, Office, MSN Messenger and Exchange.

The software maker is aware of the bogus e-mails, a company representative said Friday. It is encouraging people to go directly to its Web site for updates, instead of clicking on a link that purportedly takes them there. Once on the legitimate Microsoft site, they can click on the link that provides information on how to tell if a Microsoft security notice is legitimate.

Techniques like the Trojan horse e-mails are not new; malicious virus writers have in the past sent e-mails with attachments proclaiming to contain downloadable security updates. The Dumaru worm was one such example, Cluley said.

And in another example of attackers taking advantage of Microsoft's monthly patch cycle, malicious virus writers sent out bogus e-mails in January that claimed to come from Microsoft and that encouraged users to click on an attachment containing a Trojan horse.

The news spam e-mail started making the rounds on April 2 and continued through as late as 6 a.m. Friday PST, according to Sophos. The company noted that only 582 copies have been received, accounting for 0.04 percent of all spam that was tracked during that time by Sophos.