Windows Neptune

Microsoft Neptune is an unreleased version of Windows. It identifies as NT 5.5, and was intended to be the first consumer version of Windows NT. The project, however, was scrapped together with Odyssey (the planned successor to Neptune) and replaced with Whistler (Windows XP). As of 2018, only one build has leaked, but others may 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 (which was eventually released as Windows ME) reused files and resources from Neptune; the purpose of these files being to help bridge the gap between NT and the 9x kernel.