No internet after SpeedyPC hassle

My mother-in-law's computer has been cleaned recently after going through the Read And Run First in the Malware Removal section, but she may have been left with some unresolved issues. Before handing the computer over to me to clean for her, she tried to fix it herself by using a program called SpeedyPC. It is a program that is suppose to find and fix/clean registry problems. What it actually did was make her problems worse. After running it she wasn't able to do much of anything, like make changes to documents, change simple preferences like start menu items, and connect to the internet.

I managed to get everything working again except the internet. She uses AT&T for her ISP. They set her up with a modem that is also the router, which I have never seen. She has a laptop that uses that modem/router through wireless connection and it is able to connect to the internet. She said she was able to access the router through the desktop, but we can't figure out why she can't connect to the internet.

Device Manager does not show any issues with the network adapter (she only has one). Local Area Connection shows "Connected". IP address and DNS server is set to obtain automatically.

We have unplugged the modem/router, plugged it back in and restarted the computer.

Any ideas on what could be the problem or anything we haven't tried would be appreciated. The OS is Windows XP (Pro, I think), SP2. AT&T tech support has NOT been helpful at all.