Try deleting the user.ini file and letting the game redownload it. I once rebound esc to something other than menu, and then couldnt access the menu from ingame.
^Try turning off steam cloud and then deleting the files again. Just go to steam settings > Downloads +Cloud Tab > Then un check the steam cloud sync at the bottom. Delete the settings files and load NS2 up again.
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You can find it in: \Steam\userdata\(some number)\4920\remote
When you run the game right after that there will be a short delay while the program regenerates the file, then it will load with default bindings.
Awesome I have been wondering where the mouse sensitivity file was for a long time. Thanks.