If your iPod is set to update automatically you just need to take care when connecting back to a different computer and an empty iTunes. If it is set to automatically update you will get a message that your iPod is linked to a different library and asking if you want to link to this one and replace all your songs etc, press
*No*. Pressing "Yes" will irretrievably remove all songs from your iPod. Your iPod should appear in the iTunes source list from where you can change the update setting to manual and use your iPod on a second computer without the risk of accidentally erasing it.
You can also use a keyboard command to prevent your iPod auto-syncing with iTunes. While connecting the iPod to the computer on Windows hold down the Shift + Ctrl keys (on a Mac hold down the Option and Command (⌥ and ⌘) keys). This will stop the iPod from auto-syncing with iTunes and the iPod will appear in the source list. Wait until you are sure the iPod has mounted, and that it will not auto sync and then you can let the keys go. This may take between 20 to 30 seconds depending on your computer. Then go to Preferences>iPod and check the "Manually manage songs and playlists" box:
Using iPod with Multiple computers
Managing Your Songs Manually
