Windows Neptune (internally known as NepTune) was supposed to be the first consumer version of Windows that included Windows NT code. Windows Odyssey was going to be the successor to Windows 2000, both were merged together into Whistler, and eventually what became of all this was Windows XP.

The Windows Neptune splash screen.

The login screen has changed completely into something that resembles the final Windows XP login screen.

This is the desktop.
Other than a "Submit a bug report" option, there is no immediately discernible differences between Neptune and Windows 2000/ME.

The bug reporting application is very pedestrian and normal, nothing too different.

The start menu is basically the same as well.