Without knowing what this service agent is makes it difficult to understand where the problem lies. I never heard of any service agent or other software needed on the PC with a cable connection.
It is ATT cable you have?
This?
http://help.broadband.att.com/index.jsp?lobid=1
The problem may be with cloning in the router of the MAC address.
With my cable service what I did was register with the ISP the MAC address of the router. I suggest you call ATT and tell them you have changed your PC and heres the new MAC address (give them the MAC address of the Belkin router - it is usually printed on the bottom of the router).
However, if ATT really do need some 'agent' on the PC then this might not be a solution.
I'm afraid without knowing more the ATT service it is difficult to help.