Whenever I sync my video iPod to my laptop, a dialogue box pops up that says, "This computer isn't authorized to play songs purchased on this iPod." I've already authorized my laptop. How can I fix this?

Every time I try to sync my iPod video from late 2006 to my laptop, a dialogue box pops up that says, "This computer is not authorized to play songs purchased on this iPod." But I've already authorized my laptop. Also, I can't even purchase songs on my iPod because that version of the iPod cannot access the iTunes store. Because of this issue, I can't put any new music onto my iPod. How can I fix this? Thanks!