Apparently (I had to look this up) Neptune was a NT-based Windows (the name was actually chosen based on NT) OS that was being worked on after 2000 that never saw the light of day beyond a beta build that was released under NDA back in 2000. It pretty much ended when the people working on it were reassigned to what would become Windows XP.

ToxicOrochi said:

Apparently (I had to look this up) Neptune was a NT-based Windows (the name was actually chosen based on NT) OS that was being worked on after 2000 that never saw the light of day beyond a beta build that was released under NDA back in 2000. It pretty much ended when the people working on it were reassigned to what would become Windows XP.

It was supposed to be Windows 2000 Home Edition, but that idea was quickly squashed when Windows ME entered development. ME took elements from Windows 2000 but it ultimately was an upgraded version of Windows 98SE and prone to crash out of the box, although updates helped stability.