OEM: Once it is installed on a PC it can never be installed on another and you receive no technical support.
Upgraders: You must have a previous qualifying version of Windows installed and upgrade from it. Clean installations are prohibited.
Retail: None of the above restrictions...you receive technical support, and perform a clean install, and can transfer the license from one computer to the next.
As to Ultimate, Home Premium, etc, I suggest you consult this comparison chart.
Hope this helps,
John
Between the various denominations of Vista? I don't mean Ultimate, Premium, Business, and Basic. What I mean what's the difference between the Full on $5000000000000000 version, the Upgrade version, an the system builders version? Thanks for your help.

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