Check the sticky FAQ thread, creating overlay windows is the first thing mentioned:
http://forum.1cpublishing.eu/showthread.php?t=24782
Alternatively, go to documents\1c softclub\il-2 sturmovik cliffs of dover and find the file named ConfUser.ini, this is where those settings are stored. Move that file to another location and run the sim again, it will re-create the default settings for you if the file is missing from that location.