Periodically I get a message that I have to validate my computer. I have not changed computers since getting my iPod and downloading iTunes. It happened again this morning. It now tells me that I have validated 4 out of 5 computers. I assume that this means that after the next validation I will lose everything I have downloaded.
By validate I presume you mean authorise. Once you reach the limit of 5 authorisations you have the option to deauthorise them all in one go from your iTunes account and start over. *Just note that you can only use this option once a year*: About iTunes Music Store Authorisation and Deauthorisation