You can't unless you put it on your iPod in a specific way, its kind of confusing to explain, but they have it on the Apple website somewhere.
It's an anti-Piracy system. You can only have one 'home-base' PC etc. etc. and you can't copy songs frrom your iPod onto a computer unless you upload it in a certain way from your 'home-base PC'. Shit like that meant I had to upload all my CDs again when I got my new computer, (I used my mate's PC the first time since my old PC was incompatable with iTunes.)