Transfering music to another computer.

I have a mini(circa 2005)and I'm trying to transfer my purchased music from my desktop to my laptop. I authorized the laptop but every time I connect my iPod and try to "transfer purchases" I get a message saying files couldn't be transferred because I'm not authorized for them on my laptop. When is asks if I'd like to authorize, I try again and get the message "this computer is already authorized"

Any help?? Anyone??