What Windows version is it?
She's using Vista SP1.
And I didn't realize that you said that another computer was tested on the same modem and cable, Hanna. I guess I'll stop blaming them. Heh. That does confuse me a bit, though. I'm unfamiliar with Vista, so I don't know if it could be contributing to the problem or not. What do you think, Broni? You certainly know a heck of a lot more about Vista than I do.
I wonder if Vista Firewall might be configured improperly. Also...as your computer starts booting up, repeatedly hit the F8 key to get to the boot menu and select Safe Mode With Networking. Are you still able to connect to the internet? If so, how does it act?