I had a user with the same problem and none of the registry solutions or dictionary/language settings were the issue. I found the problem in the Add-Ins section of Word Options…
At the bottom of the screen select Manage: Disabled Addins > Go…
Another screen indicated: The items listed below were disabled because they prevented Word from functioning correctly…. Addin: speller en-us (nlsdata00009.dll)
I selected it, then clicked the Enable button and all was well again.
I restarted Word to make sure the enabled Addin remained active. So far, so good.
Before accessing or editing the registry it is reccommended that your back up the registry. Deleting or editing any files within the registry may cause certain programs not to funtion and/or cause a system failure. For detailed intruction on how to back up your registry before editing click on this link, http://support.microsoft.com/?kbid=322756
1. Go to Start, Run, and type “regedit”
2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing Tools\1.0\Override\
3. Delete the file named “Override”.
4. Restart the computer and open Word.
At this point spell check should be working.