I would recommend the steps in the article below. See the section labeled Verify that the Apple Mobile Device USB Driver is installed > For Windows XP or Vista, Windows 7, and Windows 8 > Update the Apple Mobile Device Driver.
I too have this problem with the latest versions of Windows 8.1 and iTunes.
Everything was fine until I deauthorized all computers (I sold another computer which I wanted to deauthorize). I have authorized my current Notebook several times, and it shows as being authorized, but now my iPhone and two iPods won't show up as connected devices.
I've gone through the Apple Mobile Device settings & Apple USB settings as per Apple's support pages and done resets of all devices but still nothing shows up. The only thing I haven't done is to uninstall and reinstall iTunes but I'm thinking that the problem lies with the authorization process as everything worked before I deauthorized then authorized.
I'm not one to give up but I am at a loss as to what to do next.
Your assistance would be appreciated.