I No Longer Have The Internet Due to a Windows Error

[N.W.A_Kid]

Alrighty, here goes. On my computer, I have an old 56k modem still installed, and I am currently using a cable modem provided by Cox Cable, and am sharing it with my dad's computer on a router. Everything was fine one morning, but after work I came home to find that my computer can no longer connect to the internet through the dialup modem on CoreComm, or the cable modem on Cox Cable, yet my dad still has access to the internet on his computer. Cox Cable's tech support was garbage, and they have no idea what happened. We tried several things to get the internet back. We tried the config release and renew, tried reinstalling Windows XP, we put in a completely new network card, we tried resetting the router, but nothing works. When I use the dialup, the modem actually connects, but can't access any webpages. The computer also recognizes that the cable modem's there, and everything is setup as if the LAN was working, but cannot access any pages. We've gone through so many help files and god knows what else, but nothing works at all. Anybody have any other ideas?

Windows XP home
Pentium 4 1.4 ghz
256 mb ram
linksys cable modem router
D-Link EtherNetwork card 10/100mbps PCI Adapter for XP and Mac
ECS elitegroup motherboard

If you need anymore specs, I would be happy to give them. Thanks for any ideas you guys can send!

edit: when this all happened, windows said that it recovered from some kind of critical error. unfortunately, nobody knows what the error was.