You may not realize it but when you buy music or movies from iTunes, you have to authorize it to be used on a computer and you can only authorize five computers. It's pretty easy to buy and replace computers and if you don't de-authorize the old ones, one day you will download a song and it will tell you that you have reached your five computer limit. At that point, what do you do if you don't have any of those computers anymore? Now Apple has put a fix in iTunes itself. Go to the iTunes store and sign onto the site. In your account details you'll see how many computers you have authorized. If you have authorized five computers, there will be a button to de-authorize all five computers. Push that button and you will be back to square one again. That will take care of your problem and you will be able to load songs on your computers again. Now it's really easy to take care of the authorization problem with iTunes.