Windows Neptune

Neptune was the codename for a version of Microsoft Windows under development in 1999. Based on Windows 2000, it was originally to replace the Windows 9x series and was scheduled to be the first home consumer-oriented version of Windows built on Windows NT code. Internally, the project's name was capitalized as NepTune. History 

Neptune largely resembled Windows 2000, but some new features were introduced. Neptune included a logon screen similar to that later used in Windows XP. A firewall new to Neptune was later integrated into Windows XP as the Windows Firewall. Neptune also experimented with a new HTML and Win32-based user interface originally intended for Windows Me, called Activity Centers, for task-centered operations. Only one alpha build of Neptune, 5111, was released to testers under a non-disclosure agreement, and later made its way to various beta collectors' sites and virtual museums in 2000. Other builds of Neptune are known to exist due to information in beta builds of Windows Me and Windows XP. In November 2015, a build 5111.6 disk was shown in a Microsoft Channel 9 video; version 5111 was the last build of Neptune that was sent to external testers, with the .1 or .6 after the build number stands for variant, not for compile. It is the only build of Neptune that made its way to the public. Build 5111 included Activity Centers, which could be installed by copying ACCORE.DLL from the installation disk to the hard drive and then running regsvr32 on ACCORE.DLL. The centers contained traces of Windows Me, then code-named Millennium, but were broken due to JavaScript errors, missing links and executables to the Game, Photo, and Music Centers. In response, some Windows enthusiasts have spent years fixing Activity Centers in build 5111 close to what Microsoft intended. In early 2000, Microsoft merged the team working on Neptune with that developing Odyssey, the successor 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 Windows Me in 2000 as their final 9x series installment. Some development builds of Whistler feature an improved version of the logon screen found in Neptune build 5111. Triton 

Neptune was intended to have a successor named Triton, which was to be a minor update with very few user interface changes; service packs were additionally planned for it. Triton was slated for a spring 2002 release (coinciding with Microsoft's final fiscal quarter of 2001). Triton was devised back in 1998 alongside Neptune; the only details of it within Microsoft's internal planning documentation that year relates to a deadline for added hardware support by December 2001. According to Paul Thurrott, the timeline of releases was Windows NT 5.0 (the codename for Windows 2000) for high-end workstations and Windows 98 for entry-level and mid-range PCs from 1998 to 1999; followed by Neptune in 2000 and 2001 for both workstations and consumer PCs; followed by Triton for the same target audience. However, according to Charlie Kindel, Triton was to be a version of Neptune centered on home server usage. See also 
List of Microsoft codenames

References

Neptune