Delegate Active Directory group autenticathion to a radius server

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http://stackoverflow.com – I managed my network users with an Active Directory running Windows 2008. I want to know if there is any way to delegate a specific group to authenticate trough a radius server. To be clear, the users prese CTRL + ALT + Delete to authenticate and instead of authenticate to de AD have a redirection to a radius server. The same question applies to the implementation of a strong authentication system to the windows login. Any ideas? Thanks! (HowTos)