Reset the preferences ;-
Rename the folder and then check:-
Windows Vista or Windows 7
<startup drive>\Users\<username>\AppData\Roaming\Adobe\Adobe Illustrator CS5 Settings\en_US*
Note: In Windows XP, the Application Data folder is hidden by default. The same is true for the AppData folder in Windows Vista and Windows 7. To make this folder visible, open Folder Options in Control Panel and click the View tab. In the Advanced Settings pane, select the Show Hidden Files and Folders or Show hidden files, folders, or drives option.
North America
Europe, Middle East and Africa
Asia Pacific