Chances are Outlook Express's sudden loss of the ability to check your English spelling coincided with an update to Office 2007. This Office version comes with updated dictionaries for, guess, English, Spanish and English. These dictionaries are not compatible with Outlook Express's spell checker.
Fortunately, you can turn to alternative spell checkers — or restore the old dictionary, more or less.
Get Back English Spell Checking If All Outlook Express Offers is French
To restore the English, German and Spanish as languages for the Outlook Express spell checker after installing Office 2007: