I was having this same problem but i fixed it ^_^
Mine would load up and patch but once I clicked Start it would go to a black screen then shut back down to desktop.
I decided to replace the game guard files with one from a game I know works on Windows 7. I went to my Luna Online folder and copied the GameGuard folder and GameGuard.des file and placed them into my Flyff folder, loaded it up and it worked
(image size is 1440x900 so sorry anyone who has trouble loading >.< )
I'm on Windows 7 Build 7100 32bit.
Now I'm not saying it will work for everyone but hey, atleast it worked for me? lol