I bought this game and downloaded it to my desktop PC at work, running Windows XP the other day. By editing the dosboxCannonFodder.conf file within the main Cannon Fodder directory I changed the screen resolution to 800x600. Although this made the playing area a little smaller, it looked much better and less blocky, and I was happy with it.
However, when trying the same thing when I got home on my Windows 7 laptop, it doesn't work. Regardless of what I set the resolution at it always sticks to the default. I wondered if this is because when I try to edit the file in Windows 7 it wont let me, it says 'access denied'. To edit the file I have to drag it out of the Cannon Fodder directory to the desktop, edit it, and then drop it back in.
Any ideas?
Ta!

install the game in a different folder
(e.g c:\dosgames)

Vista and Win7 are overprotective about everything that goes on inside the Program Files folder. That's why it won't let you edit the file unless you move the file someplace outside the Program Files folder.
You can either do what Qbix suggested, install your games someplace other than the Program Files folder. or if you're like me and you like to have everything installed to Program Files, run the games' installers as admin and once they're installed run them as admin aswell.
To run a game installer as admin, instead of double click right click and choose the 'run as admin' option, and to run a game as admin once it's installed tick the appropriate checkbox in the game shortcut.

Thanks guys, will give these suggestions a try when I get home.

What I ended up doing was cut-and-pasting the config file to my Desktop, changing it there, then pasting it back in the default install location.
(Well, actually, I renamed the original file, appending it with "-ORIGINAL", opened it, then saved it under the original name to the Desktop. However, after that, I cut-and-pasted it between the two locations to modify it; currently have fullscreen at false, fullscreen resolution at 1366x768, and windowed resolution kept at original.)