"The iPod cannot be ejected because it contains files that are in use..."

My iPod (20 gb, 4 gen) syncs fine, but when I go to eject it a window pops up saying "the iPod cannot be ejected because it contains files that are in use by another application."

Unfortunately, there are no files on it (that I know of) that are being used by another application.

I can't use the eject function on the task bar because I have Vista and it will corrupt the iPod. I've just been unplugging it with out ejecting, but was hoping there was a solution. Any advice?