Microsoft began work on Windows Vista, known at the time by its codename Longhorn, in May 2001, five months before the release of Windows XP. It was originally expected to ship sometime late in 2003 as a minor step between Windows XP and Blackcomb, which was planned to be the company's next major operating system release. Gradually, "Longhorn" assimilated many of the important new features and technologies slated for Blackcomb, resulting in the release date being pushed back several times. Many of Microsoft's developers were also re-tasked to build updates to Windows XP and Windows Server 2003 to strengthen security. Faced with ongoing delays and concerns about feature creep, Microsoft announced on August 27, 2004 that it had revised its plans. The original Longhorn, based on the Windows XP source code, was scrapped, and Longhorn's development started anew, building on the Windows Server 2003 Service Pack 1 codebase, and re-incorporating only the features that would be intended for an actual operating system release. Some previously announced features such as WinFS were dropped or postponed, and a new software development methodology called the Security Development Lifecycle was incorporated in an effort to address concerns with the security of the Windows codebase.
While Microsoft had originally hoped to have the consumer versions of the operating system available worldwide in time for Christmas 2006, it was announced in March 2006 that the release date would be pushed back to January 2007, in order to give the company–and the hardware and software companies which Microsoft depends on for providing device drivers–additional time to prepare. Development of Windows Vista came to an end when Microsoft announced that it had been finalized on November 8, 2006. Windows Vista cost Microsoft 6 billion dollars to develop.