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The update is the first since the company crossed the 100 million mark at the start of the new year, although Gates doesn't clarify when the company reached the newer threshold. This demonstrates that Vista continues to sell at a "very rapid" rate, according to the Microsoft co-founder.

If tracked between January and April, the number represents about 10 million copies of Vista sold per month in the first third of 2008 and signals a slight increase in the sales rate for the operating system, which averaged at just over 9 million copies sold per month in 2007. However, the sales rate is half that of the Windows update's initial results in the first two months of its launch, when it sold as many as 20 million copies per month to cater to early demand.

Most of Microsoft's Vista sales are commonly thought to stem from licenses for new computers as part of users' regular upgrade cycles rather than interruptions spurred by specific demand for the new OS. Last year, Vista was pre-installed on less than half of all new PCs despite being on sale for all but one month.

Gates' positive outlook for Vista comes as Dell and Lenovo have both said they would continue selling Windows XP past its June 30th sales cut-off date as a response to business customers, many of whom have demanded the six-year-old software remain in place to avoid compatibility problems with business software and while performance requirements remain high for the refreshed Windows platform. Microsoft has already pledged to continue supporting XP for very low-cost notebooks until 2010.