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

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

Once installed, Xombe connects to a website then downloads and installs another program — Mssvc-A — which is a Trojan horse that conscripts the host computer in distributed denial of service (DDoS) attacks against web pages, according to antivirus company Sophos.

Xombe is considered low risk by most antivirus companies, including Sophos, Computer Associates and Symantec. The program is not a worm or virus and cannot make copies of itself and can only be distributed via spam.

Those emails read, "Windows 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 attached and make sure to restart your PC after installation".

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

Antivirus companies offered updated virus definitions to spot Xombe, and provided instructions on removing Trojan programs from infected computers.

Microsoft frequently distributes security bulletins using email, 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 Centre.

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