If you want to listen to music on your friend's computer, there is a legal way to do it: use Reble. Reble is a service that allows you to share music among people that you know. You can listen to the music on their computer as if it was on your computer, and they can do the same with you. You only share with people you know (so it's a limited P2P network) which, they claim, makes this a perfectly legal use of your music.