Try these
xbox
IP=192.168.1.101
Subnet=255.255.255.0 (Leave it alone It SHOULD be set to this automatically.)
Gateway=192.168.1.1


Use these settings for your pc

IP=192.168.1.100
Subnet=255.255.255.0
Gateway=192.168.1.1

call cmd.exe and type
ipconfig
in the command prompt
This will show you the ip of the computer and the standard gateway of your network...

Are you connecting your XBox and computer to a router, or are you connecting the Xbox to the computer and attempting to use the computers internet connection? If the former, then the XBox should use the same gateway as the computer (as with Xoloriib's answer), if the latter then the XBox should use the computers IP address as the gateway and the computer needs to enable internet connection sharing (i.e. the computer is acting a bit like a router).

See this site for a guide:
http://www.plus.net/support/broadband/broadband_gaming/xbox_connection_guide.sht...