Microsoft's Whistler thief, it seems, has struck again. Another version of the company's follow-on operating system to Windows 2000, code-named Whistler, has been posted illegally to the Internet. The OS, which is tentatively scheduled to debut in March, 2001, is in development inside the walls of Microsoft. The company has been producing versions, or builds, of the OS that are only supposed to be available to Microsoft employees. The OS is not yet in public beta test. The first known copy of the OS, Build 2211.1, was posted to the Web about three weeks ago by an unknown person. However, it is likely it was a Microsoft employee, in some capacity, who first took the OS outside the company. Microsoft officials said Monday that the company is aware of the latest Whistler leak, which is of build 2223. "Microsoft is looking into recent reports that another build has been posted to the Net," a Microsoft spokesman confirmed. He would offer no other details on the build or on actions taken to investigate how build 2211 found its way to the Web. This latest build of Whistler appears to be legitimate, said Robert Stein, president of Active Network, which owns Activewin.com, one of the Web sites that first reported on Whistler's appearance on the Web. "I think it is probably the same person (who released Build 2211)," Stein said. However, "I don't think they've found them yet. Someone's just laughing at (Microsoft) right now, because it's probably not the last time it will happen." It is not clear how the newer build made its way from Microsoft's protective clutches, although it is likely that the program was saved from a Microsoft server to a CD-ROM and then uploaded to the Web. Whistler is the code name for the first full-fledged upgrade to Windows 2000 that will be based on the Windows NT kernel, rather than the Windows 9X kernel. (The Windows 9X update is code-named Millennium and expected to ship in the third or fourth quarter of this year.) What do you think? Tell the Mailroom. And read what others have said.