I had to start using a new user account recently due to a virus I caught, and then my Ipod would not sync?

Now I have restored it and then added my music back onto my itunes and it will not sync all the songs, infact it will not sync hardly any, I have only 20 songs that it has synced to my ipod when before I had around 8000, I really don't understand why this is happening, also my itunes keeps freezing whilst syncing, please help me if you could, it's urgent x