Likely, in general.. the MODEM is fine.
If your getting internet on the wireless... the credit card machines, etc work ok... than its something more on the PC.
COULD be the network card/port, has blown 
First thing i would try, just to re-fresh it.. would be to go into the networking section in control pannel, and into the adapter listing.
Right click on the network card.. and disable it.
For good measure, i like to reboot here.
Then go back in, and re-enable it..
Durring all this.. is it getting a PROPER IP from the CGN3 i wonder? Can you check the IP address?