All the game technical settings are saved in a Prefs.ini file, located under \My Documents\My Games\GalCiv2\, at least for for plain Dread Lords. You'll want to open that up in Notepad, or whatever text editor you prefer, and go down to the section marked [Video]. There are three lines there, which should read DisplayWidth=1024, DisplayHeight=768, and Fullscreen=0. Change the DisplayWidth and DisplayHeight values back to the original 1280 and 800 (or whatever you choose. You can set any values and the game will try to use them), and change Fullscreen to 1. Once saved, that should set you back to the original settings.
If you still have problems, give us a ring anytime and we'll puzzle through it. Of course, if you can include a debug.err (located in the directory you installed Galatic Civilizations II), it makes everything faster.
