Something interesting happened to Whistler, Microsoft's next generation of the Windows operating system, on its way to its second beta release. But testers aren't exactly sure yet what has changed. On Tuesday, Microsoft privately alerted Whistler testers that it is making substantial changes to the look and feel of the desktop versions of Whistler, according to tester reports. But Microsoft is taking unusually high security precautions to make sure that testers, the press and the public don't get to see the interface changes before the product -- the successor to Windows 2000 -- goes to Beta 2, which is expected to occur the week of 12 February. The Personal, or consumer, and Professional, or business, desktop versions of Whistler are expected to ship before the end of 2001, with 32-bit and 64-bit server versions slated to follow several months later. Microsoft executives have said Whistler will be the company's most important Windows release since the company delivered Windows 95 in August 1995. Microsoft sent the first beta release of Whistler to testers in October 2000. During the past few months, Microsoft has released interim beta versions of the product to its selected beta tester group. In spite of non-disclosure agreements they signed with Microsoft, testers have been fairly liberal in distributing information about the various test versions, including screen shots. But Microsoft has clamped down in recent days, to the extent that it is not allowing testers to view the code and interfaces of the most recent updates in order to keep the interface changes from leaking out, according to testers. Despite Microsoft's precautions, word is quickly leaking out about the intended changes. One tester said Microsoft is changing the Whistler desktop interface by introducing some type of "extensible shell", or graphical user interface, based on Microsoft .Net. Microsoft also is readying some type of new Windows desktop user theme, code-named Luna, as part of its revamp, according to several testers. Desktop themes are user-selectable settings that affect the look and feel of all elements of user's desktop, from the background wallpaper to the cursor. Microsoft provides a number of themes as part of Windows. Some third-party software makers, such as Stardock, also provide a variety of themes, or skins, for Windows. Microsoft officials did not respond to a request for comment on the company's interface directions. Another software company executive, who spoke on the condition of anonymity, said Microsoft has been planning on overhauling the Whistler desktop interface for several months. "Microsoft is changing all the Whistler icons, as well as the look and feel of Internet Explorer," said the software executive. Version 6.0 of the browser is included in Whistler. "When you change the theme to Luna in Whistler, it will change the themes across everything, including IE and other parts of the environment," the executive said. In recent months, Microsoft workers have been debating the role of Internet Explorer as an integrated part of its future operating systems, sources have said. Different camps within the company favour using different Microsoft browsers as the default interface for different flavours of Windows. For example, according to sources, some in the company are backing Microsoft's use of Netdocs as the default interface for business versions of Windows. Netdocs, a technology not yet in beta stage, is an integrated-knowledge worker desktop that competes with Microsoft's Office suite. Others inside the company favour Microsoft using the MSN Explorer browser, rather than IE 6.0, as the default interface for consumer versions of Windows, sources said. Sources said Microsoft had contemplated not releasing IE 6.0 as a stand-alone product and only making it available as part of Whistler. But now it sounds as if Microsoft has reconsidered that plan and will make IE 6.0 available as both a stand-alone download and part of some, if not all, Whistler versions. What's happening in the world of Windows? Find out with ZDNet UK's Windows Special. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the ZDNet News forum. Let the editors know what you think in the Mailroom. And read other letters.