Microsoft long- awaited release of Vista will likely be delayed again by at least three months, research group Gartner said on Tuesday.

The research note, released to clients on Monday, said the new Windows Vista operating system is too complex to be able to meet Microsoft's targeted November release for volume license customers and January launch for retail consumers.

A Microsoft spokeswoman said the company disagreed with the Gartner report and it was still on track to meet its launch dates.

Vista is the first major overhaul of its operating system, which sits on 90 percent of the world's computers and accounts for nearly a third of Microsoft's total revenue, since Microsoft rolled out Windows XP nearly five years ago.

Microsoft originally targeted a 2005 launch for the new Windows, then pushed the release out to 2006 before announcing in March that Vista would again be delayed to improve the product's quality.

Gartner targets a Windows Vista release in the April-June quarter of 2007, nine to 12 months after Microsoft conducts a second major test, or "beta," release for Vista during the current quarter.

"Microsoft still wants to get it out as soon as possible, but slipping from January to March is nowhere near as bad as slipping from shipping before the holidays to after the holidays," a group of Gartner analysts wrote in the report.

Gartner said Windows XP took five months to go from a second test release to the start of production, but the magnitude of technological improvement in Vista is closer to Windows 2000, which took 16 months between the second test and production.

Once production starts, it usually takes between six- to eight-weeks for PC manufacturers to load the operating system onto new computers, Gartner said.

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.