Can't get online even though says connected

Hello,

I live in an apartment that provides wireless. I was able to connect for a week and now I can't. I had been getting error messages saying that someone had the same ip address as me. And it was causing problems with aim such as getting disconnected. And I was able to connect for bit longer after that until about two weeks ago. I reformatted my laptop. Still can't connect. I bought a wireless usb card and still nothing. I am using my iPhone to connect and look for solutions but not having any luck whatsoever. I'm looking at my network sharing center and I'm connected to a networ but it stops there and there is just an annoying red x between the network and the grayed out Internet icon. And this only happens with the wireless in my apartment building. I go to my campus library and the connection is fine and I can surf the Internet. I really need help!! Please!