Credit this fix to: http://www.tech-pro.co.uk/index.html

 If you are running Windows XP and are also using Microsoft Office 97, you may find that spell checking does not work in Outlook Express 6.0. When you try to spell check a message, you receive the message "an error occurred whilst the spelling was being checked". The cause of this problem is a missing file which Outlook Express requires.

Solution: Obtain a copy of the file csapi3t1.dll and place it in the folder "Program Files\Common Files\Microsoft Shared\Proof".

Copies of this file may be extracted from the file mailnews.cab in an Internet Explorer 5.0 or 5.5 distribution set, the file win98_27.cab on a Windows 98 CD, or the file Win_10.cab on a Windows Me CD.

A copy may also be downloaded by clicking on this link.

Note: the spell checker will only work if you have a recent version of Microsoft Office. You didn't really expect Microsoft to support spell checkers in competitors' Office packages, did you?