One computer blocking others from connecting to the internet?

[ElizabethN]

I don't know how to word this but I have a wireless network. There is usually a cable from the router connected to my desktop PC. I have a macbook and always connect to the internet wirelessly. I can connect to the internet on my macbook while someone is also on the internet on the desktop PC. I can connect on the same network wirelessly while someone else connects to the network wirelessly as well. One of my roommates' laptop is junk and for some reason can't connect to our network so she took the cable that went from the router to the desktop PC and connected it to her laptop instead. Now for some reason no one can connect wirelessly onto the internet. We can connect to the network just fine as I am able to login to change the router settings, but can't get online. I think it has something to do with her laptop settings but I'm not sure. Here she is able to connect to the internet while the rest of us can't! Please help!
I forgot to add that her laptop is a Dell running Windows XP. Whenever she's not on the internet, everyone can wirelessly connect to the internet fine.