Microsoft Cairo

Microsoft Cairo is the codename of an unreleased software project by Microsoft, meant to bring next-generation technologies for Microsoft's Windows NT. The project was originally announced at the 1991 PDC (Professional Developers Conference) and later demoed at the 1993 PDC.

The Cairo project consisted of a revamped core operating system, as well as new services and server tools to ship alongside the core OS. Initially, the core OS would be based on Windows NT 3.1 and later Windows NT 3.5x, but after Windows 95 adopted the Cairo UI, it was pushed back to target what became Windows NT 4.0 instead.

The core OS focused on improving upon NT's user interface (with enhancements such as a new desktop, Smart Folders, the Explorer, drag-and-drop, and context menus) while remaining compatible with existing Windows applications. The included services and server tools focused on improving network capabilities and support for domains, networked storage, network communication, printer sharing, and distributed computing.

Cairo development continued through until 1996, after which the project was canceled. During that time many of the components slated for Cairo (such as the user interface) were instead repurposed for Windows 95, which continued the Windows-on-DOS line. Windows NT 4.0 would later ship with the Windows 95 shell and already-released Cairo components. The remaining components from Cairo would later influence future Windows projects, such as WinFS and Active Directory.

SKUs
Three different retail SKUs were planned for Microsoft Cairo:
 * Cairo Advanced Desktop would include the core OS, new user interface, and networked storage/printer sharing. It was designed as the client for Cairo Servers.
 * Cairo Advanced Server would be a superset of Advanced Desktop that would additionally include all the server roles, as well as domain services, networked messaging, and management tools.
 * Cairo Design Environment would be intended for developers to build Cairo-friendly applications, including a Smart Folders SDK, authoring tools, design tools, debugger, documentation, and Visual Basic for Applications.

Windows NT 3.1

 * DCE RPC implementation (Remote Procedure Call, allows inter-machine or inter-process procedure calls, for example, to use with distributed computing)
 * COM (Component Object Model, a standardized communication model meant to be used with RPC)
 * OLE (Object Linking and Embedding, allowing files from one app to be embeddable and editable in other OLE-compliant apps)

Windows NT 3.5x / 4.0

 * Cairo OFS (Object File System, a relational filesystem meant to replace NTFS)
 * Cairo OFS Indexing (metadata system for OFS, later turned into Content Indexing and Windows Desktop Search)
 * Cairo Domains (based on x.500 Directory Services, later turned into Active Directory for Windows 2000)

Windows 95 / NT 4.0 / Other

 * Cairo User Interface (later turned into the Windows 95/NT 4.0 Shell)
 * Cairo Messaging (based on x.400 Messaging Services, later turned into Microsoft Exchange)
 * Cairo Smart Folders (would later influence Windows 7 libraries)

Builds
As of 2020, only two builds of Cairo are known to exist, and only one (from the NT 4.0 era) is leaked.