Syncing Ipod files to another PC

Earlier I was using a Macbook Air to sync my iPod music, Videos and apps. Now since I have changed to another PC, Macbook Pro and also use a iTunes on a Windows PC at home, How can I transfer Music files and videos from iPod Touch to any of these PCs?

Am getting a message that it will erase all files from Ipod and transfer from the PC Itunes to the Ipod. I need to transfer all music & video files to the computer from Ipod. Somehow the apps have been transfered already.