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. Advertisement The program, known as "Xombe" or "Dloader-L," arrives as an executable attachment in spam e-mail messages purporting to come from windowsupdate@microsoft.com and installs itself on victim's computers when users open the attachment. Once installed, Xombe connects to a Web site, then downloads and installs another program, called Mssvc-A, which is a Trojan horse program that conscripts victim computers in distributed denial of service attacks against Web pages,...