Take Windows 2000, for example. Originally called Windows NT5, the operating system's development became 'problematic' as requirements kept being added late in its development, Gartner analysts noted. Those development changes meant the time between the Windows 2000 beta 2 release -- in August 1998 -- and the final release to manufacturing in December 1999 took 16 months.

'One should never overestimate how much Microsoft will underestimate the complexity and time needed to deliver a major new client OS,' wrote Gartner.

By contrast, Windows XP, which the research firm described as a 'relatively minor release built on Windows 2000' took only five months of polishing from its 2nd beta to its final release in October 2001. Windows 98 Second Edition and Windows Millennium Edition also came out 'largely on time,' wrote Gartner.

Vista, which includes major new security improvements and graphical user interface enhancements, was originally due out in 2004 as part of Microsoft's preferred schedule of releasing new client operating systems every three years. But its release date was pushed back several times until Microsoft committed last year to have the OS out in time for the Christmas 2006 season. Then, in March, the company announced that the release would be delayed into early 2007. But Microsoft has yet to deliver beta 2 for Vista, which will be tested by as many as two million users.

'We remain on track to deliver Windows Vista Beta 2 in the second quarter and to deliver the final product to volume license customers in November 2006 and to other businesses and consumers in January 2007,' said Sarah Williams, a Microsoft spokesperson. Even if Microsoft meets its goal for beta 2, Gartner pointed out that that would only leave five months before Vista's scheduled release to large business customers.