Vista
If UAC is enabled, this is your problem.
Somehow Vista doesn't like fullscreen (in some cases) for AoT, I've tried starting it up many times, to no avail.
What you do, is you go into your AoT installation folder (If it's in C:\Program Files, move it to C:\ before doing the following,) Open up the 'base' folder, then the 'client' folder. Now open prefs.cs with Notepad and change the following:
$pref::Video::fullScreen = "1";
Change that 1 into a 0, save the file, and try opening it again. (Make sure it's on the C:\ directory)