New iPhone 4S won't show up in iTunes (Win7)

When I plug my iPhone in, it doesn't show up in iTunes. I've followed the steps in the troubleshooting guide but nothing seems to work. Tried turning off the firewall and anti-virus, tried restarting services, tried reinstalling drivers, tried reinstalling iTunes... The iTunes diagnostic says no device is connected, but it shows up in Explorer as an iPhone.

Also, iTunesHelper is behaving weirdly. It uses up about 13% of my CPU constantly and continues to do so even when iTunes is shut down.

Any clues? 🙂