Microsoft Neptune

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A version of the Windows NT operating system
Windows Neptune login screen.png
Neptune build 5111 logon screen
Working stateHistoric, never released
Latest preview5.50.5111 / December 27, 1999; 19 years ago (1999-12-27)[1]
Kernel typeWindows NT
LicenseNon-disclosure agreement[2]
Support status

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[3] and was scheduled to be the first consumer version of Windows built on Windows NT code.


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.[2] Neptune also experimented with a new HTML and Win32-based user interface originally intended for Windows Me,[4][5] called Activity Centers, for task-centered operations.[6][7]

Only one alpha build of Neptune, 5111.1, was released to testers under a non-disclosure agreement,[2] and later made its way to various beta collectors' sites and virtual museums in 2000.[8][9] As of now, it's 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.[10][11] The centers contained traces of Windows Me, then code-named Millennium,[12] 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.[13][14]

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,[15][16] which was released at the end of 2001 as Windows XP.[17][18] In the meantime, Microsoft released Windows Me in 2000 as their final 9x series installment.[16]

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.[19]


  1. ^ Thurrott, Paul (6 October 2010). "The Road to Gold: The development of Windows XP Reviewed". SuperSite for Windows. Penton Media. Retrieved 12 August 2014.
  2. ^ a b c Thurrott, Paul. "Windows XP: The Road to Gold". SuperSite for Windows. Archived from the original on 2007-01-24. Retrieved 2010-01-06.
  3. ^ "Neptune - the new Windows". ZDNet. July 5, 1999. Retrieved 2010-01-07.
  4. ^ "Activity Centers: A Windows Me Technology Showcase".
  5. ^ "Activity Centers Preview".
  6. ^ "Microsoft Cancels Neptune, Readies 'Whistler'". betanews. January 25, 2000. Retrieved 2010-01-07.
  7. ^ "SuperSite Flashback: Neptune".
  8. ^ The Windows that Never Was… at
  9. ^ DigiBarn Screen Shots: Windows XP Neptune Build 5111 DigiBarn Computer Museum
  10. ^ "Windows Neptune/5.50.5111.1".
  11. ^ Rob Jansen. "Builds • The Collection Book".
  12. ^
  13. ^ "View topic - Neptune's Activity Center... - BetaArchive".
  14. ^ Activity Center ACCORE.DLL Partial Fix. YouTube. 13 June 2014.
  15. ^ "Microsoft combines Neptune, Odyssey into Whistler". CNN. January 27, 2000. Archived from the original on September 1, 2008. Retrieved 2010-01-06.
  16. ^ a b "Microsoft consolidates Windows development efforts". ZDNet. January 25, 2000. Retrieved 2010-01-06.
  17. ^ "Windows XP Preview". TweakTown. August 21, 2001. Retrieved 2010-01-06.
  18. ^ Christopher Jones (April 2002). "Written Direct Testimony of Christopher Jones". State of New York v. Microsoft Corporation. Retrieved 2006-01-07.
  19. ^ "Platforms Group - Desktop Three-Year Outlook" (PDF). Microsoft Corporation. February 1998. Retrieved 2010-01-08.