The error message "The custom dictionary is full. The word was not added." is not always due to custom dictionary "corruption" problems covered in MSKB article 224035 here:
http://support.microsoft.com/kb/224035
This error message can appear when you try to add a word to the custom dictionary file that contains Unicode characters and the custom dictionary text file (usually CUSTOM.DIC) is saved in ANSI format.
To correct this problem, save the custom dictionary file in Unicode format in Notepad.