CONNECTING 2 computers to a cable modem

hello guyz.... new here.... needed help wid something... i recently got a broadband connection.. i tried connecting 2 computers via a lan cable... CROSS OVER>... but the speed drop was immense.... even if my sister was checking her mail on the other computer my d/l speed would drop from 8 to 2 .....

so i tried connecting the computers through the cable modem.... but the problem here is that the computer via the ethernet port runs fine... but the internet on the one connected via USB doesnt work.... it gives a local ip .. rather than the net Ip... i hope u all get wat i mean....

The best way, and with some ISP's the only way (who only give out one IP) is to go get a router. They are very cheap these days and have the added bonus of a hardware firewall.