Networks Internet sharing 4 xbox live going wrong

Hey, i just got xbox live for christmas yeyyy, i planned to share my broadband (USB cable modem) to my xbox using a crossover cable, simple i thought. When it came to sharing it thought it doesnt work! i've tried everything.
Both IP addresses on the internet and live network connections are set to auto and im sharing the internet connection, that didnt work, it said something about a dns error. So i try manual ip addresses on the xbox live network connection, doesnt work. I tried bridging the connections which came the closest to working but then the internet on the pc wouldn't work, pleasssse help.
Thanks, chig