| Windows NT|
| Historic, never released|
5.50.5111 / December 27, 1999; 17 years ago (1999-12-27)
Neptune was the codename for a version of Microsoft Windows under development in 1999. Based on Windows 2000, it was to replace the Windows 9x series and was scheduled to be the first consumer version of Windows built on Windows NT code.
Microsoft Neptune Wikipedia
Neptune largely resembled Windows 2000, but some of the new features introduced, such as the firewall, were later integrated into Windows XP as the Windows Firewall. Neptune introduced a logon screen similar to that later used in Windows XP. Neptune also experimented with a new HTML and Win32-based user interface originally intended for Windows ME, called Activity Centers, for task-centered operations.
In early 2000, Microsoft merged the team working on Neptune with that developing Windows Odyssey, the upgrade to Windows 2000 for business customers. The combined team worked on a new project codenamed Whistler, which was released at the end of 2001 as Windows XP. In the meantime, Microsoft released another home user 9x-based operating system called Windows Me.
In the early roadmap for Neptune's development (shown in the 1998–2001 United States v. Microsoft Corp. documents) there were five service packs planned. Also in these documents, it shows that Neptune was to have a successor named Triton which would be a minor update with very few user interface changes, and service packs were planned for it. Internally, the project's name was capitalized as NepTune.