I have had my Mac for a year and seem to have a corrupted Mail program and spellchecker. If I reload the OS I know I won't have any of the updates added during the previous year. What other issues will i have and what settings will I lose?
if you mean by _reload_ reinstalling OS X, then it depends on the type of the installation. If you do an 'archive and install' then all your settings/preferences/programs will be preserved. If you do 'fresh install' then everything will be gone.
A fresh reinstall will also cause you to lose any registered software keys, make sure to back them up first if you don't want to use the hdd space of two system folders that a archive and install takes up. And you'll have to reinstall all those programs as well.