Fresh concerns are mounting that Microsoft's Windows Vista could face further delays.

The news is already worrying technology manufacturers tied to the much awaited upgrade, so much so that Microsoft's shares dropped on Nasdaq.

Windows Vista is due to be shipped to corporate customers in November, but this date may have been pushed aside.

Kevin Johnson, co-president of platforms and services at Microsoft, told financial analysts that the company will "ship Windows Vista when it is available" and that Microsoft is taking the Vista project "milestone by milestone".

The new version of Windows has already met with several delays during the five years it has been under construction.

Goldman Sachs analyst Rick Sherlund predicted that Microsoft would not begin to see any revenue from the general availability of Vista until March or April 2007.

He also suggested that the cautious comments at the analysts' meeting represented a confirmation of that forecast. "When Microsoft hesitates, that's confirmation we were right," he said.

Microsoft has consistently been this cautious in the past, an acknowledgment of its history of slipping a few months beyond initial targets for major software releases, according to Sherlund.

Microsoft chief executive Steve Ballmer told analysts at the same meeting that the "new Microsoft" is being built on four pillars.

He stated that the upgrades to the firm's core Windows and Office applications will act as drivers for growth, and will buy the company time to build two new pillars, its internet and Xbox business.