It seems you have MS Office 2007 or later version installed on your system. Outlook Express does not have its own dictionary, in fact it uses the dictionary from Office. But in MS Office 2007 and later version, it becomes incompatible for Outlook Express to use Office's dictionary. You would have three options: 1.
Get Office 2003 or any earlier version instead of 2007 or any later you are using 2.
Restore the proofing tools 3.
Or, you can download a third-party dictionary tool.
Hope this information would be helpful.