Restart your phone and your computer.
Turn off your security software.
Open the iTunes, and check for new updates – download and install them.
Occasionally, when you connect your iPhone to your computer via USB cable, for no apparent reason, some glitch may occur. Try to change the USB slot. If that does not help, unplug all the devices you don’t currently need from the USB slots.
If you saved some of the previous backups, maybe they are causing the problem. From the menu, choose Preferences, and click on the Device. You will see the backups you saved earlier, select it, and choose delete.