I just started having this problem recently after having some issues with COD4 crashing. I try to assign my mouse 5 button to an action and it keeps coming up as ALT. Which if fine if it still works, but in game when I use the button sometimes it uses the action and sometimes it pulls up the player list - kill/death page, or sometimes does both at the same time (sort of).
Any ideas? I'll try and reinstall it and see if that helps.