Windows Codename "Longhorn" was the planned successor to Windows XP. This project never released, but rather its development was reset, creating the new project Longhorn/Omega-13, which eventually became Windows Vista. The earliest builds after the reset were also known as "Codename Longhorn", causing some confusion. "Longhorn" went through many stages of development. Development started as a minor revision to Windows XP, known as "Whistler +1". Longhorn was originally intended as an interim release, a bridge between Windows XP (Whistler), and a major new version known as "Blackcomb", which eventually became Windows 7. Over time, many features slated for Blackcomb became part of Longhorn, and employees jumped ship from other parts of the company. Longhorn became a bloated, unstable piece of vapourware. The first build to leak publicly was build 3683, which contained a new theme, and also the foundations for WinFS, and Avalon, which eventually became the Windows Presentation Framework. "Longhorn" was originally intended to be an interim release, but picked up many features slated for it's successor. Stability increasingly became an issue as development progressed, and few builds were publicly released as a result. In fact, the only build to become officially available from Microsoft to the wider world was build 4074. The last known confirmed build from before the development reset is build 4093.
- After the compilation of build 4093, the development of Longhorn was reset, mainly because of the instability of the current Longhorn branch. At about 9PM, on the same day as the compilation of 4093, Microsoft stopped development of Windows Longhorn, and started fresh using Windows Server 2003 Service Pack 1 as a codebase. Development of Longhorn continued, albeit many features originally slated for inclusion were delayed or dropped to provide a more realistic operating system. Examples of this include WinFS, and what would become Windows Powershell.
- See Windows Vista for information on the post-reset (Omega-13) builds.