Uninstall the driver using Revo Uninstaller (free version, left column). When you uninstall the driver, in Revo Uninstaller be sure to use the Advanced Mode so you can get anything that may be leftover in Windows Registry.
http://www.revouninstaller.com/revo_uninstaller_free_download.htmlWhen you're done with that, reboot windows. Then redownload the drivers (just to be sure the previous downloaded files did not get corrupted), and reinstall them.
After you have reinstalled the drivers:
1: Turn off your phone.
2: Plug the phone in to your PC via USB
3: Turn on the phone
Theoretically this should force an update to the drivers you just installed and everything should work properly. If not, try steps 1-3 again only this time use a different USB port. If this fails, then I'm unsure how to help. :(