According to a report that was posted at Information Week, most of the business customers of HP downgrade their computers from Microsoft’s Windows Vista operating system to Windows XP.
Chris Flores, who is the official blogger of Microsoft Vista, said that over 180 million Vista licenses were sold. Information Week notes that this news raises questions about the sales claims of the Vista operating system since businesses cannot buy just a Windows XP system without buying a license for Windows Vista.
Dell and other computer makers are also shipping these “downgraded” systems.
Windows XP was officially retired by Microsoft last month.
Source: Information Week

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so retired that you can still buy it as a upgrade to vista, and as an oem from many online distributors. i.e. tigerdirect.com
As long as XP is better people will still want it over Vista.