Did you make sure you have the latest version of iTunes?
Have you tried to reset your iPhone? That is, by holding both the home button on the front and the power button on the top right; it will bring up a red slider bar that will allow you to slide to power off.
One other thing that might casue issues is security software. Did you install any new security software? If so you may need to make an exception for iTunes (if it allows that) or turn it off while you sync.
Also did you try making an exception for iTunes in your firewall?