The iPod is set to automatic mode by default. This means that whenever you connect your iPod to your computer, the iTunes application automatically synchronizes the music on your computer and iPod. If you connect your iPod to a different computer, you are prompted to replace the current music library on your iPod with the music library on the new computer. To keep the current music library on your iPod and still sync it with the new computer, you must set your iPod to manual mode.
Connect your iPod to your computer with the USB cable.
Launch iTunes, then select your iPod from the left pane of the window in the Devices section.
Click the "Summary" tab, then click the "Manually Manage Music and Videos" check box in the Options box.
Click "OK" on the confirmation dialog box, then click "Apply."