Doesn't matter if you're synced. If you want, unplug your iTouch. But, all you do is go into iTunes Store on your computer, and at the top right there is a sign out button. Or, under Store in the drop down menu at the top left of iTunes, choose Sign Out (second from the bottom).
Then log in on your own account or stay logged out, but just sync your iTouch and you're good now.