Microsoft Corp's efforts to release its Windows Vista operating system in January is likely to be stalled. Chief executive Steve Ballmer refused to commit to the January release date, but insisted the firm was confident Vista would be released to consumers next year. 'At this stage, I think the most important thing is that we ship a high-quality release. You know, we didn't make Christmas. That was the big milestone,' he said. When Microsoft announced in March that Vista would not make its Christmas release, it said the product was on track to be available in January. Vista's release for corporate volume licensing customers was due in November. Research firm Gartner said wide availability of Vista was not expected until at least the second quarter next year, as Microsoft had a history of missing new operating system release targets. Vista was originally planned as a 2004 release. But the final decision on releasing Vista apparently rests on the product-refresh timetables of the major computer vendors, such as Dell, Hewlett-Packard and Lenovo. 'Right now, we're going to the big guys, [asking them] when is the best time for a machine change with the new operating system,' Mr Ballmer said. He said Microsoft's computer hardware partners were relieved about Vista's missed delivery date. 'It would have disrupted their supply chain. They only change their machines at certain times of the year,' he said. Gartner has forecast a minor impact on Vista's delay to computer shipment growth this year. It said the January release would reduce unit sales growth about 0.6 percentage point, equivalent to 1.1 million fewer units. Vista's launch next year was expected to add 0.9 percentage point of growth, equivalent to 800,000 new units. Jack Lee, the general manager of Microsoft's equipment manufacturing division in China, said: 'The new release date has minimal impact on mainland PC makers since their busiest season will be the Chinese New Year winter holiday in January to February.' About 90 per cent of personal computers are shipped with Windows, which is used in more than 750 million computers worldwide and contributes about 30 per cent to Microsoft's annual revenue.