The revamp is promised to be the biggest update to the operating system since Windows 95 launched in 1995.

The preview showed a new desktop design, more visual ways to organise information, and faster searching.

The full version is expected in December 2006, but a preview, or beta, could be out in the summer, he said.

"We just gave people a glimpse to show them it's an awfully big deal," Mr Gates said in an interview after his speech at the Windows Hardware Engineering Conference in Seattle.

Longhorn, whose December 2006 release could still shift after previous delays, should offer users more security features, much richer graphics technology, and faster performance.

The way people can switch from window to window and the way files are presented on a PC with Longhorn is reminiscent of Apple's operating system, according to analysts.

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"Our key goal in terms of Longhorn is that it be the highest-quality release we've ever done," Mr Gates said.

"At every stage of the way we're going to listen to feedback, so it's possible some of these milestones will change and we'll choose to put more time into things."

During his speech, Mr Gates added that the Microsoft developers had concentrated a great deal on making sure people's data was safer on PCs.

Previous versions of Windows have been criticised for being too vulnerable.

Longhorn will make use of cryptographic keys stored in chips built into a PC for the first time. Such encryption features are usually kept as data on a hard drive.

Having encryption keys actually on a chip makes it harder for data to be compromised through hack attacks.

Some PC manufacturers have been offering computers with security chips for some time.

The last version of Windows operating system, Windows XP, was released late 2001.

Microsoft's operating system runs on more than 90% of desktop PCs in the world.

Mr Gates announced that a more powerful, advanced version of the current Windows system, called Windows XP Professional x64 Edition, would also be available for business users, from next month.