Windows Longhorn was a codename for a planned release of Windows, which was initially expected to ship in 2003. It was meant to be a major update to the Windows XP operating system, with a focus on security, reliability, and usability. Longhorn featured a new desktop environment, called "Aero," which was designed to be more visually appealing and user-friendly. The operating system also included new features like SuperFetch, which improved system performance by preloading frequently used applications.