Try XLan to get play an online/systemlink gamesXLan Setup Guide
A tip to never be detected again is to manualy change the Default Gateway of the xbox 360.
this will stop any internet activity from the xbox but it will allow you to use streaming media and other tools
it is quite easy to do.
Goto your network setup.
Assign a manual IP Address and set you Default Gateway to your PC`s IP Address.
Press test setting and test Xbox Live.
it will sign you out, once the tests have started quickly cancel them.
Then go back then back again.
Now look at your Network setting they have saved ok.
Now you will not be at risk of ever going onto Live at all.