As the WinHEC 2005 developer conference kicks off today in Seattle, much of the focus will be on Longhorn, with Microsoft publicly previewing its next-generation Windows release for the first time since April 2004. But Microsoft Chairman Bill Gates will also herald in the third decade of Windows, showing off a tiny tablet PC.

The prototype mini-tablet, which was commissioned by Microsoft, is as thin as 10 sheets of paper and weighs only 1 to 2 pounds with a built in camera and 6-inch screen. The device contains a battery that can last all day, and is priced between $500 and $800.

However, the tablet is intended only to show off what the future Windows computers could look like; Microsoft doesn't expect such devices to reach the market for another two years.

In his keynote address Monday, Gates will also show off some portable PCs designed for Longhorn that are closer to delivery. One will be a new Tablet PC from Acer with a side-down keyboard, and the other a laptop with a display on the outside shell. The external screen is designed to enable basic functions like playing music and checking e-mail with the lid closed.

On the software side, Longhorn build 5048 was distributed to WinHEC attendees late Sunday. Because the release is intended for developers and hardware makers working with the new platform, Microsoft has included few bells and whistles. Build 5048 does sport the new Aero "Glass" theme, although Longhorn's Desktop Window Manager (DWM) is not active.

DWM is the new Windows interface engine built using Avalon, Longhorn's graphical subsystem. Eventually, it will enable advanced visual capabilities that Microsoft is expected to show off during WinHEC, but DWM also brings with it hefty hardware requirements.

Microsoft is preparing hardware makers for Longhorn's expected debut late next year, already launching a new logo program to designate "Longhorn Ready" PCs and a premium logo for hardware that takes advantage of Longhorn's new functionality.

Longhorn is far from feature-complete, but Gates will unveil a new open document format, code-named "Metro," in Longhorn that will utilize XML. The idea is to enable viewing of documents without requiring the application used to create them. Such native Windows functionality could eventually render Adobe's PDF obsolete.

For now, Adobe can rest easy; Longhorn is still in its early development stages, with Beta 1 not expected until at least June. That isn't stopping Microsoft from rallying the troops however, as Redmond on Monday launched a contest to promote Longhorn-designed PCs as part of its "Start Something" campaign.