Windows Neptune
| Windows Neptune | |
|---|---|
| Version of the Windows NT operating system | |
 | |
 Neptune build 5111's desktop | |
| Developer | Microsoft |
| Working state | Historic, never released |
| Latest preview | 5.00.5111 (kernel reports) 5.50.5111 (version in many files) / December 27, 1999 |
| Marketing target | Consumer |
| Kernel type | Hybrid (NT) |
| License | Non-disclosure agreement |
| Support status | |
| Cancelled | |
Neptune was the codename for a version of Microsoft Windows under development in 1999. Based on Windows 2000, it was originally intended to replace the Windows 9x series and was scheduled to be the first home consumer-oriented version of Windows built on Windows NT code. Internally, the project's name was capitalized as NepTune.
Neptune largely resembled Windows 2000, but some new features were introduced. Neptune included a logon screen similar to that later used in Windows XP. A firewall new to Neptune was later integrated into Windows XP as the Windows Firewall. Neptune also experimented with a new HTML and Win32-based user interface originally intended for Windows Me, called Activity Centers, for task-centered operations.