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Neptune (sometimes stylized as NepTune) is the codename for a canceled version of Microsoft Windows scheduled for release during the year 2000, which would have been the first consumer version of Windows based on the NT codebase. At first, it was supposed to succeed Windows 98, although this later shifted to Windows Me due to delays. Major user experience enhancements were planned to ship with Neptune, including Activity Centers, a new task-based user interface that involved a group of applications, each focusing on a different area of functionality, that grouped common tasks together in a common web-based interface. The management of multiple users was also improved in Neptune with the introduction of a new login screen and user manager. The new interfaces were primarily implemented using web technology, often using the new Mars framework.
Another key point of the Neptune project was to experiment with new experiences that didn't require the user to manually save their work. Some of this effort is visible in the available build, which enables hibernation by default and requires the user to take extra steps to fully shut down the computer. Fast Startup, a feature that captures the system state immediately after a normal boot and then restores it on further boots, was also introduced.
The project was worked on through most of the year 1999, until it was ultimately scrapped in January 2000. The Neptune team merged with another team developing Odyssey, a business-oriented successor of Windows 2000, to work on what would later become Windows XP. However, many of the project objectives did end up inspiring some of the new features of the versions that followed, with Windows XP introducing an evolved form of Neptune's new Start menu design as well as certain bits of the task-based interface. The Metro interface introduced with Windows 8 also shared many common goals with the Neptune project, including the Activity Centers' focus on typography. Windows 8 also introduced Hybrid Boot, which is very similar in principle to Neptune's Fast Startup ability.
A minor update to Neptune codenamed "Triton" is mentioned in internal Microsoft documents from 1998, which was planned to be released in March 2001, 12 months after the initial release of Neptune. According to the documents, Neptune would also receive several Service Pack updates while Triton would have been in development.
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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.