The reset occurred as Microsoft's development staff had lost focus on the project as a whole and what was required to be done in order to bring it to market.
Development on the OS started in May 2001 and went through two unique development cycles separated by a development reset in 2004.
Windows Longhorn was the pre-release codename for Windows Vista and was the successor to Windows XP and Windows Server 2003 (built from NT 5.2 codebase).
