40 million = 2 months PC unit sales

239 million PCs sold last year (from Gartner), that's an average 20 million a month, so 40 million is 2 months worth of average PC sales. Presumably the sales figures include the Christmas Vista vouchers.

So Jan Feb, March, April, and the Christmas vouchers.... results in 40 million units. About 50% takeup on new PCs.

Is this correct? Vista's shipped on 50% of new PCs?

That would fit the story from Feb that shrink wrap sales of Vista were about a third less in revenue compared to XP (higher price, lower volume, overall worse):

http://www.crn.com/software/197006491