I recently upgraded from Office 2003 to Office 2007, and for some reason the spell-checker stopped working (not that I need one, of course ) - no matter what I typed, Word didn't react. After checking all settings and contemplating downgrading, I found the following solution at http://forums.techarena.in/showthread.php?t=734509 that worked like a charm:
delete the following registry key:

HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\ProofingTools\1.0\Override\en-US
You never know