Microsoft started work on their plans for Windows Vista ("Longhorn") in 2001, prior to the release of Windows XP. It was originally expected to ship sometime late in 2003 as a minor step between Windows XP (codenamed "Whistler") and "Blackcomb" (now known as Windows "Vienna"). 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 with improving the security of Windows XP. Faced with ongoing delays and concerns about feature creep, Microsoft announced on August 27, 2004 that it was making changes. The original "Longhorn," based on the Windows XP source code, was scrapped, and Vista development started anew, building on the Windows Server 2003 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. After "Longhorn" was named Windows Vista, an unprecedented beta-test program was started, involving hundreds of thousands of volunteers and companies. In September 2005, Microsoft started releasing regular Community Technology Previews (CTP) to beta testers. The first of these was distributed among 2005 Microsoft Professional Developers Conference attendees, and was subsequently released to Microsoft Beta testers and Microsoft Developer Network subscribers. The builds that followed incorporated most of the planned features for the final product, as well as a number of changes to the user interface, based largely on feedback from beta testers. Windows Vista was deemed feature-complete with the release of the "February CTP," released on February 22, 2006, and much of the remainder of work between that build and the final release of the product focused on stability, performance, application and driver compatibility, and documentation. Beta 2, released in late May, was the first build to be made available to the general public through Microsoft's Customer Preview Program. It was downloaded by over five million people. Two release candidates followed in September and October, both of which were made available to a large number of users.
While Microsoft had originally hoped to have 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, so as to give the company – and the hardware and software companies which Microsoft depends on for providing device drivers – additional time to prepare.
Through much of 2006, analysts and bloggers had speculated that Windows Vista would be delayed further, owing to anti-trust concerns raised by the European Commission and South Korea, and due to a perceived lack of progress with the beta releases. However, with the November 8, 2006 announcement of the completion of Windows Vista, Microsoft's most lengthy operating system development project came to an end.