I can understand that the greenPC might not support a serial port or internal Modem, but the Live CD can run on my laptop. I don't ever use the dial up modem any more but I would think that there should be some drivers for a modem to run under debian ubuntu
Not speaking from experience of course, just guessing
In any event the OP needs to supply some more detail what kind of ATT service does she have BellSouth/att DSL or ATT Dialup ??
I'm not American, so I don't have a clue what "at&t service" is. Do you mean you are using dial up on a POTS line? In that case sorry, but gOS does not support slow internet connections (using modems), only connections that are made though the network (ethernet) cable.