Windows Neptune

Neptune was the codename for an unreleased version of Microsoft Windows. It identifies itself as NT 5.5 (although many files in build 5111 have the version "5.50.5111.1", and this is even the true version in the kernel, the kernel actually reports "5.00.5111.1" to match the subsystem version in its own PE header), and was intended to be the first consumer version of Windows NT. The project, however, was scrapped together with Odyssey, which was the planned next major version that would succeed Neptune, and replaced with Whistler in early 2000. As of 2021, only one build has leaked, but others likely exist.

Major enhancements were planned to ship with Neptune, such as Activity Centers, which were HTML and JavaScript-based applications meant to replace Win32 apps. A new log-on screen was also developed that would later resurface in Whistler. Early builds of Millennium reused files and resources from Neptune; the purpose of these files being to help bridge the gap between NT and the 9x kernel. A lot of these features were reportedly reminiscent of Metro in Windows 8.

According to antitrust documents, a minor update to Neptune called Triton was planned to be released 12 months after the initial release of Neptune in March 2001 presumably to bridge the gap between Neptune and Odyssey.

According to an internal Microsoft document, Neptune would be released in March 2000 with Service Pack 1 being planned to ship in May 2000 and Service Pack 2 in September 2000. Service Pack 3 would be released in January 2001, Service Pack 4 in July 2001, and Service Pack 5 in March 2002. That would make Neptune the 2nd place in most Windows Service Packs right behind Windows NT 4.0.

List of known builds
Although only build 5111.1 is currently available, there is some evidence for other builds as well, mostly from file versions in various Windows Me builds.