itunes sync problem

hello,

last week i sync my iphone 4s in itunes on my windows 7 emachines laptop, it works. but now its not syncing anymore it always say " The iPhone "Ernesto's iPhone" could not be synced for purchased items that are on this iPhone. To authorize this computer for purchased from the Itunes Store. choose Store > Authorize this Computer."

After i authorized the computer, and i'll try to sync my iphone again then it will back to.(authorize this computer)

what's wrong? is it my iphone? nor my itunes?

thanks.