What you need -
A computer with the Library you want to move (I've done it with Windows, not sure if it works the same way with Mac, but it should), new computer you're planning to move to (of course), an external storage media (like a hard drive, DVD, or your iPod with disk use enabled) which can accomodate your entire Library.
Get Started -
Install iTunes on the new computer. Set it so that it doesn't automatically add any audio files from the computer when installing it. Close the app and move to the old computer.
Plug in the storage device you are going to use, and first navigate to the folder containing your Library (it's by default in the Music folder on Vista and 7, and in the My Documents/My Music folder on XP). Move that folder onto the storage media. Start iTunes and deauthorize the computer from your Apple Account (if applicable) by going to Store (on the menu bar) -> Deauthorize Computer...
Now connect your storage media to the new computer you are moving to. Move the iTunes folder to the Music folder, replacing the folder created automatically when installing iTunes. It's always a good choice to let a copy of the Library stay on the storage media you used, as backup.
Start iTunes, but press and hold the Shift key when starting it. If you haven't started iTunes after installing it, it will show the Terms and Conditions and start normally. Close it and start it again, with the Shift key pressed. A different window will open with a couple of choices. Click the button that says 'Choose Library'. In the dialog now open, navigate to the iTunes folder you copied from the external drive or whatever, and double click the file that says 'iTunes Library'. Voila, you have the iTunes Library on the new computer, with all the music, movies, podcasts and whatnot.
You're up and ready. Change the preferences to your heart's content and authorize the computer (if applicable), by going to Store -> Authorize Computer...
Here you are, you just transferred your iTunes Library to another computer. Congratulations. Bye, until the next post, and play fair...
