Microsoft shares suffered their biggest intraday fall in nine months yesterday, a day after its chief executive said analysts' revenue estimates for the new Vista operating system appeared to be "overly aggressive".

Steve Ballmer's comments, made at a presentation to Wall Street analysts, came two weeks after the launch of Vista, Microsoft's new operating system. It is intended to replace Windows as Microsoft's core profits earner and is its most important new product since the launch of Windows 95 almost 12 years ago.

On Thursday, Mr Ballmer warned that Vista would not lead to a big increase in Microsoft sales in the coming fiscal year. "You shouldn't think of a huge surge in fiscal year '08 relative to '07," he said.

Roger Kay, an analyst at Endpoint Technologies, said the comments may have been a way for Microsoft to dampen expectations for the coming quarter, which typically marks a seasonal low for consumer PC sales.

"Most consumers aren't buying PCs right now, and corporations are serious when they say they are not going to start adopting Vista until 2008," he said.

Microsoft's shares fell as much as 2.7 per cent in early trading yesterday. By midday, they were trading down 2.2 per cent at $28.81.