Just an odd chance there is a registry error.....
You may want to try checking these registry entries for MS Office:
HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing Tools\Custom Dictionaries
There should be a "Default" String value with no data and a String Value named "1" with the data "CUSTOM.DIC".
Values are repeated here for Office XP and some vers of 2000:
HKEY_USERS\S-1-5-21-1935655697-926492609-839522115-1003\Software\Microsoft\Shared Tools\Proofing Tools\Custom Dictionaries
If the HKEY_USERS.... item above is not found on your system, please try a search for "Custom Dictionaries" from the root of HKEY_USERS to locate.
Good Luck!