The release of Windows Vista will knock Apple's market share slightly for the next few months, Piper Jaffray warns.

Analyst Gene Munster points out that while sales of Vista boxed versions have failed to meet expectations, sales of PCs with the new operating system installed have climbed, threatening to impact Apple's share of current sales.

The analyst expects Mac market share to shrink from 2.5 per cent in December to 2.3 per cent in March as a result of the pent-up demand for Vista.

Despite the statistical glitch, Munster told clients: "We believe the halo effect is still strong, and PC demand driven by Vista will fade in June and Mac market share will rebound."

Munster maintains confidence that Apple will continue to gain market share across the year, and posits that the move to Vista may be a tipping point at which some Windows users may choose to move to Mac.

"We believe widespread computer upgrades could lead consumers to evaluate their options, which may allow Apple to sway buyers toward the Mac platform," he wrote.

The imminent release of Mac OS X 10.5 'Leopard' will also drive the Mac market, he says: "With the release of Leopard, Mac market share will benefit from upward pressure from slight pent-up demand."

Since Mac OS X 10.4 'Tiger' was released, Apple's installed user base has climbed from 16 million to 20 million users, he said, predicting 40 per cent of Mac users will upgrade to the new operating system within 12 months.

He predicts Leopard will attract 2.6 million sales in the June quarter, contributing $130 million to Apple revenues.

Munster surveyed 50 Best Buy stores to arrive at his conclusions.