Although it's unclear what is included and what not on XP's sales figure, for Vista's positive outlook, Microsoft has not only included copies sold at retail shelves and licenses bundled with new PCs, but also numbers for those that bought Windows XP before Vista's launch and applied for the free upgrade. Microsoft has already commented, this last was not a driving factor for the 20 million.
PC makers also say that they are encouraged by early results for Windows Vista.
"Overall we've seen a pretty good reaction to the release of Vista," said Kenneth Walker, chief technologist at PC maker Gateway.
Perhaps another fact to consider in comparing XP and Vista sales, is that the PC market has nearly doubled since year 2001 (136m units) when the former was introduced. Last year about 227 million computers were shipped according to numbers from IDC.