Um I'm on a router with another person. If one person is doing illegal thing can they determine which machine is doing it and can tell whether we are using a router? They probably know my ISP and username and password for my internet usage. (Don't think anything bad towards me >.<)
It is quite possible that if they wanted to they could track it back through the router to the specific IP address on the actual PC used.
Tell the person doing the illegal things to start looking into software like peerguardian and others.
If it were my connecting the one thing i would NEVER let people do is download movies and music as the RIAA and MPAA are on a rampage right now.
My router has a set of controls where I can stop connections to certain websites from certain machines. It's a home brew machine but I'm running belkin software on it.
If this is your connection you have the right to control what it is used for. You can lookup your router here http://portforward.com/ and have a look at the options you can use to stop this sort of activity. You can close p2p ports and restrict access to any sites from any machine on your network. (if you have a very good firewall it is even possible to block transfer across it of certain types of files to a particular address)
Of course you must remember to set a secure admin password as soon as you apply any restrictions to stop the unauthorised user just changing the settings back.
If you are concerned about abuse of your account with your ISP then change your password, and tell them to get their own account and connection and take responsibility for their actions.. Then it's not your problem in any way.
As for finding stuff out about you.. Even clued up home users can find out all kinds of things about your setup. I know how to find from your ip which ports are open, what operating system you are running, and by sniffing the packets in and out of your connection the mac numbers of your hardware.. and by certain means, I can see exactly what you are downloading/uploading, legal or otherwise. Passwords for websites, and any and all unencrypted data in or out. Nasty eh? That's what the RIAA and MPAA do.. They log ip's using certain sites (by running one of their machines on the torrent network and participating in the swarm or whatever).. kind of "caught red handed" there as the tracker will tell all other machines in the swarm your public ip.. not the one behind your router.
It's your connection, and therefore your responsibility ultimately for any illegal activity it is used for
