You might need to uninstall iTunes and all its components as described in this article: http://support.apple.com/kb/HT1923?viewlocale=en_US&locale=en_US, then reinstall the latest version of iTunes. You must follow this article (installing all the components in the prescribed order) or it will not work. Note: uninstalling iTunes only removes the application. It won't touch your iTunes library or data.
Start -> control panel -> administrative tools -> services
Look for Apple Mobile Device.
Stop service, start service, plug in device
Still not working?
Follow step 4 here: http://support.apple.com/kb/TS1538