Connecting to xbox live with internet sharing

I have a laptop with vista premium. It is connected to the internet
wirelessly through a linksys router that is connected to my comcast modem. I
am able to media share with the xbox 360 with no problems. I am not able to
sign on to xbox live however and the xbox fails any test for connecting.
Can anyone help. I've tried the the procedures listed by microsoft. No
success.