When you run ipconfig /all on a Windows 7 computer you will typically see the word "Preferred" beside the auto-generated link local IPv6 address for the computer. Does this mean that Windows prefers IPv6 over IPv4 for communicating over the network?
Not usually. What "Preferred" generally means in this context is simply that the IPv6 address has been verified to be unique on the link and is therefore allowed for normal usage of communicating over the network. And if both your IPv4 and IPv6 are displayed as "Preferred" then the preference between them as far as network communications is concerned is determined by the prefix policy table as defined in RFC 3484, see http://www.ietf.org/rfc/rfc3484.txt for details.
Mitch Tulloch is a seven-time recipient of the Microsoft Most Valuable Professional (MVP) award and widely recognized expert on Windows administration, deployment and virtualization. For more tips by Mitch you can follow him on Twitter or friend him on Facebook.