Problem with pre-owned Iphone 4-Please help

I have a previously owned (unlocked) Iphone 4, and it is showing the previous owner's messages instead of mine.

When I first switched it on it showed new texts for me, but it changed when it connected to Itunes. Is this something to do with the fact that I synched it before when it was still locked to the previous owner? Is Itunes/my pc/the Iphone confused???