Early alpha builds of Longhorn, the next Windows version, appear to have leaked to the Web, according to a group of Windows enthusiasts who are now using the product. The Longhorn builds, which are variously identified as "alpha" or M3 ("milestone 3"), show a side-mounted task pane, or sidebar, similar to the dashboard debuting this week as part of MSN 8. But in these alpha quality builds, the user interface is very much based on Luna, Windows XP's user interface, and does not include the 3D video layer that will adorn the final Longhorn product. Still, as an early peek at the company's post-XP work, the Longhorn alphas--if real--are interesting.

"It's an alpha, Milestone 3 build, which means anything could still change," Arnaudt of WiNBETA (www.winbeta.org) told me. "Currently, it has an early version of WinFS, which I disabled--as you can with any service--because it takes up 100 percent of the CPU. But it has nice new GUI enhancements and overall I'd say it's pretty impressive for an alpha. The sidebar is still very buggy, and doesn't interoperate very well with IE 6.05, causing sidebar crashes every so often."

I will say this. The releases appear genuine, though I'll have to see a build with my own eyes to be sure. In the meantime, information about the leaked code suggests that Longhorn will include some of the following enhancements over XP:

- A task pane with quick links to multiple desktops, a clock, applications and shortcuts, and other widgets. This task pane can be mounted to either side of the screen or hidden, and optionally combined with the taskbar. The Start menu is basically identical to that in XP.

- A softer, Luna-based UI called "Plex" that might be compared to the "Watercolor" UI from early XP builds. That is, I expect it to be replaced by something more compelling further down the road.

- New animated versions of system folders like My Documents, My Pictures and My Music.

- New applications and activity centers based on Longhorn's "Avalon" API, which appears to be an XML-based successor to the Win32 API used by today's Windows applications. An example of this new type of application is Display Properties, which presents a vastly different UI than today's simple dialog. In the Longhorn alpha screenshots I've seen, Display Properties is a single window, task-based application, similar to the User Accounts application in XP. Another example is the new My Hardware application, features a collapsible and graphic view of your PC's hardware, divided into categories such as Audio Devices, Input Devices, and so on.

- An updated version of Internet Explorer, probably called IE 7 or IE .NET by the time Longhorn ships. Like early XP builds, the Longhorn alpha features a slightly modified version of the current IE version as a placeholder; in this case, IE 6.05.

- The WinFS file system, which is only partially implemented at this stage. This SQL-based file system lets you run ad hoc queries from the Explorer UI, in a manner similar to the Smart Playlist Editor in Windows Media Player 9.

So is it real? I honestly don't know, but if this is fake, it's the most elaborate fake I've ever seen, with multiple willing participants. I'm still holding out for code, and some sort of corroboration from one of my Microsoft sources. But in the meantime, this looks like the real deal.