Apple's integration of the iPod with iTunes has made downloading and managing music quick, convenient and safe. However, iPods need to be "authorised" (or "registered") to play and transfer content to and from individual computers. The good news is, if need be, you can "de-authorise" (or "unregister") your iPod and switch it to another computer in a few easy steps.
Know the authorisation range of your iPod. Individual iPods can be authorised to play content downloaded from iTunes on up to five computers, so you may not have to de-authorise your iPod. If you still need to de-authorise your player, go to step 2.
Open iTunes and connect your iPod to the computer from which you wish to de-authorise it. It may take a few moments for iTunes to recognise your iPod. Once it has, iTunes will show your player in the left hand column.
If you are running a newer version of iTunes, go to the top menu and choose "Store" and then "De-authorise computer." If your version of iTunes is older, you may access this function from the button marked "Advanced."
Click on "De-authorise computer for Apple account" and enter your personal account information. Wait a moment, and your iPod will now be de-authorised from this machine. You are now free to authorise it for use with another computer.