Security companies are warning Internet users about a new Trojan horse program spreading via spam e-mail and masquerading as a Windows XP software update from Microsoft.

The program, known as "Xombe" or "Dloader-L," arrives as an executable attachment in spam e-mail messages purporting to come from [email protected] and installs itself on victims' computers when users open the attachments.

Once installed, Xombe connects to a Web site, then downloads and installs another program, called Mssvc-A, which is a Trojan horse that conscripts victim computers in distributed denial of service (DDoS) attacks against Web pages, according to antivirus company Sophos.

Xombe is considered a low risk by most antivirus companies, including Sophos, Computer Associates International and Symantec. The program is not a worm or virus and cannot make copies of itself. Instead, it is distributed using spam e-mail messages.

Those messages read, in part, "Window (sic) Update has determined that you are running a beta version of Windows XP Service Pack 2 (SP1). To help improve the stability of your computer, Microsoft recommends that you remove the beta version of Windows XP SP1."

Recipients are told to "run the file winxp_sp1.exe in attach (sic) and make sure to restart your PC after installation," according to CA, Sophos and others.

Sophos said it has received several reports of the Xombe Trojan from customers.

Anti-virus companies offered updated virus definitions to spot Xombe Friday and provided instructions on removing Trojan programs from infected computers.

Microsoft frequently distributes security bulletins using e-mail, but never includes software updates as attachments, according to the company's website.

Most Microsoft software updates are made available through the Windows Update, Microsoft Office Update or the Microsoft Download Center, the company said.