This is a long shot, but googling has failed me. I recently updated my PC to windows 10. After I did this, I tried playing Portal through steam. But the controller configurations are all fucked up. The joystick used to walk is also controlling the looking, so if I step forward, I also look up. Make sense? The other joystick still only controls the looking around. Anyone have any ideas?
I found this on the Steam Community. Their problem was different than yours, but their method of restarting everything might help you if it's a software/config issue.