They are included with iTunes. Make sure you are running iTunes 10.7 or higher (required to sync the iPhone 5). Then go through the troubleshooting steps in this article: iOS device not recognized in iTunes for Windows.
If it still isn't recognized, uninstall iTunes and all of it's components by following the appropriate guide: Windows XP or Windows Vista/7, then reinstall itunes. This will not touch your iTunes library or data.