Why wont my computer authorize my ipod to sync?

when i connect my ipod to my pc through usb it gives me a pop up message saying that the computer is no longer authorized to play purchased items that are on the iPod and it gives me 3 options to "cancel" "authorize" and "dont authorize" and it says that if i dont authorize then all the songs i purchased with drm will be removed. if i click authorize then put in my account info and unknown error comes up with the number (-42408) next to it and because of this my ipod wont sync anymore!!!! please help!!!