For Windows Vista users
I have windows vista and this is what I did to access Xbox live through my laptop.
1st - connect the laptop to the xbox via a crossover cable (i bought mine at office max for 20 dollars)
2nd - go to Control Panel, then click on Network and Internet, then click on manage wireless networks, then click on adapter properties, then click on the sharing tab and enable ICS(internet connection sharing) now in that tab, i checked every box even though i didn't know what they really meant, so u might need to do the same
Also, make sure your xbox isn't turned on until steps one and two are done.
I hope this helps, it did for me