I am having the same problem. I would like to have a full screen because and looks better and recording videos is better. Where is the fsx.cfg that I can delete to make it work again?
Go to your main file (im not sure what its called but it holds your documents folder, pictures folder, videos folder etc.) and it should have your name on it.
At the top (this is for vista so im not sure where it is in XP) but find folder options or folder and search options.
Then go to view in the folder options and look for display hidden files and click yes..it will be or it should be on no.
Then click okay and a dimmed folder called AppData should show at the top.
Then once you enter find the roaming folder (there should be a few folders maybe 3)
After Roaming find Microsoft
Then click FSX
and there you will find the .cfg just called FSX.
Just drag it to the desktop to keep if anything goes wrong but make sure it not in that FSX folder.
Then load the game and it will reset all your settings so it might act like its a brand new game when it boots up.
It wont delete any of your addons so dont worry.
I hope that helps 8-)