sync &erase... boo boo

Hi, i recently changed computers PC and moved my itunes to it.

When i first hooked up my iphone 4s to it i must have hit the button so it would NOT sync and erase and not show that prompt again!!

Now when it try to sync it wont do it because my iphone is too full to add what i want.

Is there anyway to get it to make it sync and erase or to show me the prompt again?