You may have tables that were exported and imported between formats over the years which is causing the tables to lose their intended format Some of them may be set to latin1_swedish.
set your character set to UTF8 and collate to utf8_unicode_ci
Example:
ALTER TABLE $table CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci
EDIT:
research "SQL Error 1366"
Also read through this, since you are running your forums using MICROSOFT WINDOWS.
http://stackoverflow.com/questions/1168036/how-to-fix-incorrect-string-value-errors
