Ever wanted to move your music library from one computer to another and keep all your ratings, playlists, etc. in iTunes intact? You can! This hint is structured for going from a Windows box to a Mac, although there is no reason it wouldn't work in the reverse or from a Mac to a Mac or from a Windows computer to a Windows computer. Each variation is just another minor tweak that you should be able to figure out on your own.

1. Consolidate your iTunes Library. This is the default setting on the Mac, but not so on Windows. Go to Advanced » Consolidate Library. iTunes will copy all of your music into your "some_folder/iTunes/iTunes Library/" folder.
2. Open iTunes on the computer you will be transferring the music to. Make sure iTunes runs, etc. If there is anything in this library, it won't be for long. (You can copy these songs in later, but you won't be able to keep their ratings, etc. For the adventurous, seek out SyncOTunes, a somewhat abandonware project that may help you.) Open iTunes' Preferences and make sure that the options "Copy Music to iTunes Library" and "Keep Library Organized" are checked. They'll come in handy later.
3. Close iTunes. Drag its iTunes folder to your Desktop. On the Mac, it is found at "~/Music/", on Windows, "My Documents/My Music/".
4. Move the iTunes folder that contains the library you want into the spot you just vacated.
5. Re-open iTunes. There is a good chance you will see a dialog pop up that reads "Upgrading Library" if the versions don't exactly match. In fact, going from an older version to a newer version is sometimes better as iTunes does some magic to the XML file that contains its library on the upgrade cycle which can weed out old junk.
6. If the iTunes folder that you dragged to your Desktop contained anything you wanted, you can now open it up, open the iTunes Library folder, and drag those folders (which contain the music) into iTunes to copy them into iTunes. Otherwise, trash that whole iTunes folder; you don't need it. (The one on your Desktop, silly!)

Follow those six steps and you should have successfully have migrated your iTunes Library from one computing platform to another, just like that!