Shares in Microsoft dipped sharply today after the world’s largest software developer sought to dampen expectations over Vista, its new operating system.

Steve Ballmer, the Microsoft chief executive, said that analysts were being "overly aggressive" in assessing Vista’s contribution to earnings over the next year.

Brent Thill, the Citigroup analyst, wrote in a note: “Our biggest takeaway from the event is that [Microsoft] is clearly trying to temper the exuberance in some Street models over Vista.”

The comments marked an about-turn by Mr Ballmer, who had previously forecast that take-up of Vista would be five-times faster than that of Windows XP, its predecessor, which was released in 2001.

In fact, market research published this week suggested that Vista had actually sold half as many units as XP in its debut week, which began on January 30.

Mr Ballmer said: “You shouldn’t think of a huge surge in fiscal year ’08 relative to ’07."

Canaccord Adams, the analyst, trimmed it forecasts for Microsoft’s 2008 financial year after Mr Ballmer’s remarks, reducing its revenue projections $600 million to $56.12 billion (£28.8 billion).

Shares in Microsoft fell as much as 2.5 per cent, their biggest drop in nine months, to $28.76. The stock had gained nearly a third in value since June.

Costing an estimated $10 billion to develop and constructed from 50 million lines of code, Vista took five years to deliver.

Accounting for the lion's share of Microsoft's profits, its success is seen as key for the company as it fronts up to younger rivals such as Google.

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