I've had this happen a few times, it's caused by a bad restore file normally.
1. Disconnect your iPhone from the computer.
2. Make sure the time and date on the computer is correct.
3. In iTunes, go to Edit > Preferences > Devices.
4. Under Device Backups, select your device in the list.
5. Click Remove Backup.
6. Close iTunes.
7. Reopen iTunes. Reconnect your iPhone.
It should make a new backup and that should solve the problem, if not pm me.