How do I play the game in full screen/windowed mode?
The simplest way to toggle between full screen and windowed mode is to hit alt-enter. You can also change whether AC runs in full screen mode or in windowed mode by clicking on the Settings button in the lower right corner, click on the Config tab, then check the Full Screen option to play full screen, or uncheck it to play in windowed mode. Be sure to click the Apply button or it will revert when you click on something else.
There are occasions where you may need to change this setting without being able to get into the game (for example if playing in full screen mode and your firewall needs to update it's settings you may not be able to see the popup and it will the loading will stall). Do this by going to your My Documents folder, and in the Asheron's Call folder open the file UserPreferences.ini using Notepad or another text editor. Near the end change the setting to either FullScreen=False or FullScreen=True, save the file and restart AC.