Windows Longhorn

The early development stages of Longhorn were generally characterized by incremental improvements and updates to Windows XP. During this period, Microsoft was fairly quiet about what was being worked on, as their marketing and public relations efforts were more strongly focused on Windows XP, and Windows Server 2003, which was released in April 2003. Occasional builds of Longhorn were leaked onto popular file sharing networks such as IRC, BitTorrent, eDonkey and various newsgroups, and so most of what is known about builds before the first sanctioned development release of Longhorn in May 2003 is derived from these builds. After several months of relatively little news or activity from Microsoft with Longhorn, Microsoft released Build 4008, which had made an appearance on the Internet around February 28, 2003. It was also privately handed out to a select group of software developers. As an evolutionary release over build 3683, it contained several small improvements, including a modified blue "Plex" theme and a new, simplified Windows Image-based installer that operates in graphical mode from the outset, and completed an install of the operating system in approximately one third the time of Windows XP on the same hardware. An optional "new taskbar" was introduced that was thinner than the previous build and displayed the time differently. The most notable visual and functional difference, however, came with Windows Explorer. The incorporation of the Plex theme made blue the dominant color of the entire application. The Windows XP-style task pane was almost completely replaced with a large horizontal pane that appeared under the toolbars. A new search interface allowed for filtering of results, searching for Windows help, and natural-language queries that would be used to integrate with WinFS. The animated search characters were also removed. The "view modes" were also replaced with a single slider that would resize the icons in real-time, in the list, thumbnail, or details mode, depending on where the slider was. File metadata was also made more visible and more easily editable, with more active encouragement to fill out missing pieces of information. Also of note was the conversion of Windows Explorer to being a .NET application.

Most builds of Longhorn and Vista were identified by a label that was always displayed in the bottom-right corner of the desktop. A typical build label would look like "Longhorn Build 3663.Lab06_N.020728-1728". Higher build numbers did not automatically mean that the latest features from every development team at Microsoft was included. Typically, a team working on a certain feature or subsystem would generate their working builds which developers would test with, and when the code was deemed stable, all the changes would be incorporated back into the main development tree at once. At Microsoft, several "Build labs" exist where the compilation of the entirety of Windows can be performed by a team. The name of the lab in which any given build originated is shown as part of the build label, and the date and time of the build follow that. Some builds (such as Beta 1 and Beta 2) only display the build label in the version information dialog (Winver). The icons used in these builds are from Windows XP.

At the Windows Hardware Engineering Conference (WinHEC) in May 2003, Microsoft gave their first public demonstrations of the new Desktop Window Manager and Aero. The demonstrations were done on a revised build 4015 which was never released. Several sessions for developers and hardware engineers at the conference focused on these new features, as well as the Next-Generation Secure Computing Base (previously known as "Palladium"), which at the time was Microsoft's proposed solution for creating a secure computing environment whereby any given component of the system could be deemed "trusted". Also at this conference, Microsoft reiterated their roadmap for delivering Longhorn, pointing to an "early 2005" release date.