Easiest way it to clear the cache (I just delete the cache folder itself) and then verify the files in Steam, this will revert your game to the default install.
You can rename the '..\Documents\1C SoftClub\il-2 sturmovik cliffs of dover\' folder (don't delete it as all your setting are in the .ini files and you may want to copy them back into your 'clean' install), this folder will be recreated when you run CloD next.
If you truly want to re-install here is how I did it:
http://forum.1cpublishing.eu/showpos...69&postcount=3