Microsoft Corp.'s much- and long-awaited release of the upgrade to its flagship Windows operating system will in all probability be delayed ince more by at least three months, said research group Gartner Inc. last Tuesday.

The research note, released to clients 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.