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Did you register or anything then?
I would consider starting from scratch.
Oh and made sure you have a crossover cable between PC and Xbox?
As for the IP - it is unique to each user online - therefore, if you disconnect and recconect you'll have a different IP.
Two people (unless sharing an internet connection) can not be online with the same IP. So giving mine, for example, would be useless.
I 'reckon' the problem is actually somewhere in the PC settings, you shouldn't normally have to manually configure the Xbox settings.
All I can do is post this link which again, you've probablly already been to http://www.xbox.com/en-GB/live/start/connect/sharedpc /troubleshooting.htm
I really hate it when things like this happen with PC stuff because I end up changing so many settings and stuff that I just loose track of what I have and haven't tried.
Good luck, hopefully someone else can help better - I know adrian uses/used ICS and he's like a networking god type person - I'd post this in the PC Forum as well 'cause they've helped me out a few times with Live problems.