Lost Internet Connection

emachines T3025
3000+ AMD Athlon XP Processor
160 GB Hard Drive
CD-RW 48x Max Write
Nvidia GeForce 4 MX Integrated Graphics Card
DVD 16x Max Read
10/100 Mbps Ethernet

I have Roadrunner, and two computers networked to a router. I thought if I switched to my old router, I could get faster downloads, since it would be easier than port forwarding. I switched routers, and lost internet connection on the other computer

Usually, when I plug a network cable into the computer, it has this “trying to connect” symbol then there’s a yellow ! in the tray. I go on Firefox and it says it can’t display anything

But at Best Buy, it connected to the internet. Best Buy didn’t help me. emachines suggested I try different things on the phone, and none of those worked. I tried plugging in different ports in the router, getting a new cable, uninstalling the network driver, system restore, using a different ethernet card, and plugging a USB cable into the modem to the computer. When I plug in the USB, there’s a red X in the tray

What should I do?