Sounds like it was allowing you to access the full screen when you first installed it? I have to ask the following since you are on Win 7.
When you installed did you -
Turn UAC off during the install?
Install in a directory outside of program files?
Right click the setup.exe and click "Run as an administrator" on the resulting dialog?
After installing, did you right click the game short cut, click Properties, click the Compatibility tab and checkmark "Run this program as an administrator"?
On the same tab checkmark "Disable visual themes" and "Disable desktop composition"?
The fact that you were denied access to the config.div makes me think you did not get a proper install, especially if you did not do the above. I would right click the game short cut as described above and checkmark the three items on the Compatibility tab and try the game. Sometimes that is all that is needed to gain full access for the game exe.
If that gives no joy than temporarily disconnect from the internet and temporarily disable your firewall and anti-virus program. Play the game and try setting the res in the config.div file (located in the DD install directory, open with notepad) to 1024 x 800 and attempt to create a character. If you can edit the file, set the res and create a character you need to make the div.exe a trusted program in your firewall and maybe your av program.
If none of the above helps and you did not do the steps during install that I mentioned above . . . I reluctantly suggest you uninstall the game (backup your saves first) and delete any files / directories that may remain after the uninstall.
I would turn UAC off and create a directory on the C drive, not inside the Program Files directory. I use C:\Games. Make sure you right click the setup. exe and click "Run as an administrator" on the resulting dialog. On the first dialog of the GOG installer, click the Options button in the lower left corner and point the install to the directory you created on the C drive. After the install, right-click the game short cut and checkmark the items mentioned above.
That should give you a clean, full install. I turn UAC back on afterward myself. With an install outside of the Program Files directory you should be able to edit the config.div file and set the res to the highest res your monitor supports. I would try the 1024 x 800 as it is a nominal size which seems to work better with DD than the sorta odd resolutions that some monitors use as a default. Win 7 security and UAC can sometime result in access problems for the game and that seems to be what you are experiencing. If none of the above helps . . . post back . . . =)