Cause - New Proofing Tools in Office 2007
Users of Windows XP will find that upon installation of Microsoft Office 2007, spell checking in Outlook Express will only be available in French. This is due to the new proofing tools used by Office 2007; Outlook Express can't use them because it's designed to use older tools from previous Office versions.
There is a workaround for this, though it may have unexpected results if Office Add-in software is used (see below). Take a previous version of Office and install just the proofing tools (be sure to choose the custom install option). The old proofing tools can be pulled from a previous Office suite or application - including Front Page, Publisher or standalone versions of Word, Excel, or Outlook. Once installation has completed, restart Outlook Express. Browse to Tools, then Options, then click the Spelling Tab to ensure it is configured for English.
Unintended results may be experienced once installation of the earlier proofing tools is completed; trying to install an add-in under the Genuine Advantage program may fail because the older proofing tools are not activated. If the add-in is required, there are steps to take prior to completing the workaround to avoid potential issues later on. Either install the proofing tools from Office 2000, which require no activation, or use a 3rd party program for the same function. If the workaround does not resolve the issue, uninstall the old proofing tools, install the needed add-in, then reinstall the proofing tools.
If the older proofing tools are installed after installing Office 2007, be sure to run Office diagnostics as a safety measure. Also note that Windows update will probably want to install updates for the older version of Office because of the proofing tools. Choose to ignore these updates, as only updates pertaining to Office 2007 are required.
Microsoft's official solution to this issue is to download Windows Live Mail, which is available for download here. Moving forward, Outlook Express will not be included with Windows 7. If unable to use the above solutions and/or Windows Live Mail, download a third party spell check program from the Internet. Microsoft's official take on the situation can be seen here.