Windows Neptune is a version of Microsoft Windows that was planned to be the home consumer edition of Windows 2000. Early in development, the project was cancelled and was merged with "Odyssey" to form "Whistler" - which eventually became Windows XP.

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

The Neptune project was eventually cancelled because of clashes over previously promised features and a desire to do something new with the product.

Features

Windows Neptune was supposed to feature an HTML-based user interface code-named Forms+, Universal plug-and-play (UPnP) connectivity, Bill Gates's WinTone vision, and the NT kernel used in Windows 2000 (since renamed as the Windows Engine).

Some of the changes introduced in the Neptune build, such as a rudimentary firewall for Internet and network connections, were integrated into Windows XP.