My pc is telling me that i have to authorized my computer before i can download previous purchases. What do i do?

I just got a new laptop. After I downloaded iTunes to my computer, I plugged my iPod Touch into the laptop. Every song that I manually downloaded onto my iPod through a CD was deleted, along with all my apps. All the songs that I had bought in the past through the iTunes store on my iPod was still there. I had to go back to my old computer to download everything that I had lost. Now I want to use iTunes on my new laptop and all the songs are there, but I can't play them through my laptop. My laptop says that it is not that I have to "authorize" my computer before I download previously purchased items on to my computer. I have no idea what that means. HELP PLEASE.