Question: I have a bunch of music in iTunes on my old computer. How do I transfer my iTunes library to my new computer?
Answer: There are two ways to do this: 1) by manually copying your iTunes music files to an external hard drive and then manually copy them to your new computer, and 2) using a program like iPod 2 iTunes or MediaPilot that let you copy music from your iPod directly to another computer.
I'll cover the manual route first, which is a little harder and requires that you have an external hard drive, and then I'll show you the easy way using just your existing iPod and an inexpensive copy of iPod 2 iTunes.
Manually copy your iTunes library to another computer using a external hard drive
Plug in your external hard drive to your computer. Open Windows Explorer and go to your iTunes music folder. On Vista it's C:\Users\your.username\Documents\My Music\iTunes\iTunes Music. On Windows XP it's under Documents and Settings.
Copy the iTunes Music folder to your external hard drive, and then plug the hard drive into the other computer.
Make sure iTunes is installed on the other computer, and just copy the contents of the iTunes Music folder from your external hard drive to the new computer (in the same location as the old computer). iTunes will recognize the files the next time you run it, and you'll be good to go.
Automatically copy your iTunes library to another computer with third-party software
A much easier option if you have all the music on your iPod is to use a third-party program that will copy music from your iPod to your computer (basically using your iPod as an external hard drive).
There are a couple of programs that you can use to upload music from your iPod to any computer running iTunes.