okay, the only thing i can think of is that maybe you have added songs from another persons computer onto your ipod??
are you using a network between your computers and other computers? or just your computer and the internet? a smart thing to do would be to create a WEP key for your network, this is just a password, that is something like 38 characters long, this would stop any future intrusions that may prove more sinister than just someone accessing your shared folders.
either you have put someone elses songs onto your ipod from their computer or your network has been accessed by an unauthorised person..... maybe get a computer technician to configure your network and security?