Windows Neptune is a version of Microsoft Windows that was shown as technology demonstrations of a possible home consumer edition of Windows 2000. Early in development, the project was cancelled and the "Whistler" project was formed that eventually went on to become Windows XP.

In April 1999, it was formally announced that Windows Me was going to be the final product in the 9x family. One Neptune build did make it to beta testers (on Dec 27, 1999) and showed off some early work on the new logon screen ("Welcome" in Windows XP) and an activity center-based User Accounts control panel. These beta testers speculated that Neptune was slated for release, in 2001 or 2002, as the first NT-based consumer Windows offering.