You can't buy a legitimate operating system on the market right now, but you can purchase a "legitimate" Windows Vista beverage: a specially customized version of Lemon-Lime Talking Rain soda by Microsoft, with a flavor of lemon and sweet orange.
The packaging of the soda also features the Vista logo and an internal corporate URL for more information about Vista. This promotional method is quite interesting, I wonder how effective it will be.
Lemon and sweet orange? I prefer green tea.