What I'd do is the following:
1. Reinstall your iTunes library once more - this time including all its components and services exactly as described here:
2. Restart your PC
3. Change your iPhone USB cable and make sure neither the USB slot on your computer nor the USB connector on your iPhone are damaged. Also, avoid using USB hub devices but connect straight to your computer
4. Restart your iPhone as described here:
5. Make sure WiFi sync is disabled for your iPhone:
5. Run iTunes and connect your iPhone to your computer.