So I have just searched for this file in 'my Computer' and found 13 copies, All different versions and for different games. That is probably why you're other games work fine, they all have their own version in their game folder

When you tried reinstalling the game, did you go into Common and delete the L4D2 folder completely or just reinstall straight away? Reason I ask is some times 'bin' folders do get left behind and thats where this little creep hides.