I copied and pasted this from another discussion regarding a similar question:
Go into Programs and Features
Uninstall:
iTunes
Bonjour*
Apple Software Update
Apple Application Support
Quicktime*
iCloud*
Apple Mobile Device Support
* I didn't have to uninstall Bonjour, iCloud or Quicktime to fix my issue but it won't hurt anything to uninstall them.
Reboot. Download a fresh copy of iTunes. Right click on the exe file and select Run as Administrator if you have the option if not just Run it. Should fix the issue.