If you've purchased them through itunes, you can plug it in and it'll ask you to transfer all purchased items from iPod to your PC.
There are various programs that will take all of your music off of your ipod, the issue is, iPod puts them in its own special organisation, which I can't see any logic to, to be honest...
Open up your music folder on the iPod in windows explorer and see for yourself. It's all available to you, just not named the same way...