Like ChimpZealot said - you need the MAC address of your Xbox360 registered with IT. If they are unwilling to register the extra MAC for you (assuming you've already registered your PC) then simply go into your Xbox360's network settings and set the MAC address field to use your PC's MAC address. The only downside to doing this is that you can only use 1 device at a time (xbox or pc) but this was what I did to get on xbox live when I went to college. Good luck!
p.s. if you dont know how to get your PC's MAC address do the following. Start > run "cmd" by typing cmd into the search box and hit enter > type "ipconfig /all" into the console that pops up
Your PC's network adapter MAC will be listed along with IP address and other network essential information