Question Regarding new DSL connection

[ripdog]

We're finally going to make the leap to high speed. I'm a computer retard, my wife is fairly literate but doesn't think she'll be able to set up the new connection for multiple computers. We'll have a single IP address, company supplied modem, and a firewall router. Will the ISP be able to detect a second computer? One of her work friends set up a wireless network that worked for several months, then the ISP cut off the second connection. Are there any configurations that will keep the second connection cloaked?