Broadband Won't Work

Hi:

Tremendous frustration. I have Windows XP Home Edition. I have used Cox broadband for 18 mos with no problems. Suddenly the machine (CompUSA brand P-IV 2.6 mz) wouldn't connect to the internet via the cable modem. Since then I have spent hours on the phone with Cox tech support and they have given up, basically. What we have tried are:
-I have replaced the cable modem
-I have had the network card checked. CompUsa was able to connect to cox with my computer from the store.
-I disabled and uninstalled my virus software and firewall
-I had the computer swept for viruses and spyware
-I have tried connecting with USB using two different cable modems
- I have done a system restore to before the problem occurred
- I have checked all the TCP/IP settings to match Cox's specifications
-I have allowed the system to remain off for >24 hours as Cox said this might "reset" the connection. Didn't
-I am able to access the service with a different computer and the ethernet cable.

I am completely stumped.

Please.....any ideas?

Scott Young
Virginia Beach