I did not try it yet but it is proposed on a different board to:

1. Verify files via steam
2. empty content of cache folder of Clod (also removing the files in the shader folder inside the cache folder without removing the shader folder itself iirc); folders: .\Documents\1C SoftClub\il-2 sturmovik cliffs of dover\cache
3. install the patch
4. start the game
5. exit the game
6. restart the game
7. play

for ati card owners following file must be removed too:
Logo.wmv in folder ..\Steam\SteamApps\common\il-2 sturmovik cliffs of dover\parts\core\GUI

Instructions courtesy FG28_Kodiak