limited or no connectivity

Hey would someone be able to help me im trying to connect to a cable modem through a rj-45 that has just been put in. when i plug it in alot of packets are sent but no more than 2 are recieved. the strange thing about it is i know it cant be the connection hardware because my brothers vista laptop plugs in and worx perfectly. I did just come home from a college that makes me switch my proxys but i dont think that matters plus im pretty sure ive switched them to the proper settings anyways. a prompt aswer would be appreciated

thanks