VoIP question

Hello,
If I am asking this question in the wrong newsgroup, accept my apologies
and please point me to the correct one.
Here is my situation:
I signed up for ATT's VoIP phone service. I am currenting using Road
Runner cable Internet service which connects to my Win 98 computer with
a USB cable. The telephone adapter requires a ethernet card which I can
install and Widows will detect.
There is a ethernet connection on the cable modem. When I connect the
ethernet cable between the modem and the computer, I lose my Internet
connection. I receive error various error messages saying that there is
no communication between the two.
I have been on the phone for hours withtech support to no avail.
Any help would be appreciated.

Thank you,
....Will