ex_matelot,
You may access your iTunes account from as many computers as you wish, but you may only authorise up to five computers from the point of view of the actual intellectual property content (i.e. music files etc.).
In order to do so, all you need to do is install iTunes as normal, and then either run/restore a backup or just copy the content across to the appropriate directories. You can do that through whatever means you like.... USB drive, burn a CD/DVD, share a folder over crossover, LAN or WLAN.
See here also