What is the best way to join an internet pc to domain?

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http://serverfault.com – I need to connect a PC that has alredy an internet connection via onboard lan to a windows domain. This domain has no internet connection because data is confidential. I need to do this because i need send some files from domain to another place (by autorized personal only). At this day this is made via modem, but it's too old, slow and the software that do the task isn't working on windows 7, but for my boss this is better than connect the domain to internet. I need to figure out how to connect this computer to the domain network without letting non autorized users share files between domain (HowTos)