Help.Longhorn - What is it?
http://www.hyperwrite.com/Articles/helplonghorn.aspx

Self, Tony
HyperWrite
2004

Abstract:

The Help platform for Microsoft Windows is changing once again. Since 1995, Microsoft HTML Help has been the standard for Help systems for Windows applications, but the release of the next generation Windows operating system in 2005 will see a brand new XML-based Help platform. It is currently known as Help.Longhorn, or "Longhorn" Help, or sometimes as Help3 or TrésHelp.