Microsoft Cairo

Microsoft Cairo is the codename of an unreleased project by Microsoft, originally meant to build 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 project consisted of revamped core operating system components, as well as related software technologies and tools that would run alongside the revamped OS.

Initially, the core OS was to be based on Windows NT 3.1, but after Windows 95 adopted the Cairo UI, it would target what became Windows NT 4.0 instead.

Development continued through until 1996, after which the project was cancelled. During that time many of the components slated for Cairo (such as the user interface) were instead repurposed for Windows 95, which continued to be a Windows-on-DOS release. Windows NT 4.0 would later ship with the 95 shell and previously released Cairo components. Other components from Cairo would later influence future Windows projects, such as WinFS and Active Directory.

Only one known leaked build of the Cairo project (as a whole) exists, using a build of Windows NT 4.0 (SUR) as the core OS.

Windows NT 3.1

 * DCE RPC implementation (allows inter-machine or inter-procedure 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 4.0 (Leaked SUR build)

 * Cairo OFS (Object File System, a relational filesystem meant to replace NTFS)
 * 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)