Microsoft Cairo

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

Timeline
BetaArchive user jimmsta theorized that, contrary to what was stated in the timeline above (that OFS was never released):

SKUs

 * Cairo Advanced Desktop was designed as the Client for Cairo servers. It would have included the core OS, new user interface, and networked storage/printer sharing.
 * Cairo Advanced Server
 * Cairo Development Environment

Builds

 * 3.1.297.1
 * PDC 1993 build - shown at the Professional Developers Conference held in December 1993
 * 4.0.1175.1