I doubt I've understood your setup. Please tell me if I'm correct: you have a modem, you'd regularly connect a computer to in order to get internet access. You also have a router, another computer, and an xbox. You are trying to share access to the internet between the computer and the xbox. Your setup should be something like this:
Code:
[COMPUTER] [XBOX]
\________/ \____/
| |
\________ __________/
| |
| |
__________|___|__________
| LAN PORT | | LAN PORT |
|----------- -----------|
| |
| R O U T E R |
|_________________________|
| W A N P O R T |
|_________________________|
|
_____|_____
| M O D E M |
|___________|
|
______|____
| INTERNETS |
| |
/ \
/_ _ _ _ _ _ _ \
..........................
.... .. .. ..
Do you like my ascii art? [] yes [] no [] maybe