Details are emerging about Microsoft's plans for Longhorn, its forthcoming Windows operating system upgrade.

A list of the key subsystems upon which Microsoft is said to be basing Longhorn has been posted on a Microsoft Longhorn forum. The list is based on a document – a 1,000 page "Book of Longhorn" - that is circulating internally at Microsoft.

According to the posting, the book is divided into the following seven sections:

– Aero, the 3D-rendering user interface;

– Avalon, the core set of application programming interfaces (APIs) for handling graphics and presentations;

– Indigo, the next release of Microsoft's Web-services infrastructure that will underlie the OS; .NET Remoting + MSMQ + ASMX + .NET Enterprise Services (a k a COM+);

– WinFS, the Windows File System data-store that Longhorn will borrow from Microsoft's SQL Server "Yukon" database. It will be able to store XML and metadata in a single place;

– Real-time communications and speech. The instant-messaging, P2P technology and the core speech API that will be built into the platform;

– Trustworthy Computing and Security, which, in Longhorn's case, will consist largely of the Next-Generation Secure Computing Base, or "Palladium" code;

– Fundamentals: Integrated workflow capabilities; rights management; perhaps even the good old .Net Framework.

The company is expected to reveal more about the OS upgrade during its Professional Developers Conference (PDC) taking place in Los Angeles from 26-30 October.

Details of the seemingly Apple-inspired user interface Aero are unlikely to be previewed at the PDC event, but attendees will learn more about Avalon, the OS engine that underpins Longhorn's UI. Developers at the show will be told how to take advantage of Avalon in their applications.

The new OS is not expected to be released until 2005.