Instructions
In-Game
Click the "Options" button, located to the right of the text boxes on the opening "World of Warcraft" screen.
Select the "Graphics" option from the entries displayed on the left side of the Options window. Examine the array of menus displayed. Find the Display Mode pull-down menu, located near the top of the Options window.
Open the pull-down menu. Select the option labeled "Windowed (Fullscreen)." Click the button labeled "Apply," followed by the button labeled "OK." Log into your account, if desired.
Settings File
Open the folder that contains the "World of Warcraft" application. Open the folder labeled "WTF" and locate the file labeled Config.wtf. Save a backup copy of the Config.wtf file to an alternate location.
Open WordPad, TextEdit or any other text editor application. Click the "File" menu and select the "Open" option. Navigate through your file system to the contents of the WTF folder. Double-click the file labeled "Config.wtf."
Browse the contents of the Config.wtf file and locate an entry containing the words "SET gxWindow." Change the entry's number value to "1." If the Config.wtf file does not contain this entry, add "SET gxWindow" to the end of the file, without the quotation marks. Input "1" at the end of the entry, with the quotation marks included.
Locate an entry containing the words "SET gxMaximize." change the entry's number value to "1." If the "Config.wtf" file does not contain this value, add the entry using the method described above. Save your changes and exit the text editor; the "World of Warcraft" application will use the "Windowed (Fullscreen)" setting next time you launch the program.