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A new Trojan horse program appeared on the Internet Friday, and security experts are worried that its disguise will help it infect a large number of computers rather quickly.

Known variously as Xombe and Downloader, the program is attached to an e-mail that purports to come from Microsoft Corp. and supposedly delivers security updates to the user’s computer. Experts haven’t yet analyzed all of the Trojan’s capabilities, but once activated, it downloads another executable file from a remote Web site. This file also appears to be a Trojan, which downloads and installs a third program. The third file can vary in name, but immediately begins requesting pages repeatedly from a Web site with a Russian domain, possibly as a denial-of-service attack.

Xombe arrives in an e-mail from the address with a subject line of “Windows XP Service Pack 1 (Express)-Critical Update.” The attachment is named “winxp_sp1.exe.” According to an analysis of the program done by Computer Associates International Inc., the body of the message reads:

“Window Update has determined that you are running a beta version of Windows XP Service Pack 1 (SP1). To help improve the stability of your computer, Microsoft recommends that you remove the beta version of Windows XP SP1 and re-install Windows XP SP1. If you cannot remove the beta version, you should still reinstall Windows XP SP1.

Windows XP SP1 provides the latest security, reliability, and performance updates to the Windows XP family of operating systems. Windows XP SP1 is designed to ensure Windows XP platform compatibility with newly released software and hardware, and includes updates to resolve issues discovered by customers or by Microsoft’s internal testing team.

The maximum download size is approximately 3 MB, however, the size of the download and time required may be less for computers that have had updates previously installed.

To minimize the download time needed for installation, setup will only download those files which are required to bring your computer up to date. Windows XP SP1 includes Internet Explorer 6 SP1. Anti-virus software programs may interfere with the installation of Windows XP SP1. Please disable anti-virus software while installing the service pack.

Just run the file winxp_sp1.exe in attach and make sure to restart your PC after installation will be completed.”

The Trojan has not infected many machines thus far, and both CA, based in Islandia, N.Y., and Network Associates Inc. have Xombe listed as a low threat currently.