Fiddler, Outlook Express uses the spell-checker that is incorporated in MSWord.

Try going into the "tools" menu while you're inside a message. Look for a "spelling" entry. If it's grayed out, you won't be able to use that spell-checker, and will need to install MS Word.

I've been using a great little piece of freeware, Tiny Spell.   It has no spyware. Unlike other spell-checkers, it can also check your spelling on the fly from any application.

Teresa