New computer wont connect to network/internet at cust. loc.

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I have just recently built a Win XP machine for a customer who has comcast cable internet. They have a Mac setup and have no issues. I installed a router and right then it gave me issues. Neither computer could connect with the router. I switched it back to direct connection with the modem. The Mac connected fine. The WinXP system would not connect. I checked the LAN and it would not acquire an IP address. I did an ipconfig /release/renew and it hung on renew. Then gave me an error. The LAN indicator on the task bar said limited or no connectivity.

I then took the PC back to my own how where I also have comcast broadband. The PC connected fine.

Back once again to the customers house and once again, limited or no connectivity.

Called comcast, they told me that I should either pay for a second service so the new pc can have its own IP or buy a wireless router because it seems to be the NIC. It cant be the NIC can it? It works at my house...

Any help would be greatly appreciated.