Assuming you have this problem in Word 2007:
1. Verify settings in Word 2007:
a. Open the document that has this problem. Press Ctrl+A to
select all of the text.
b. Click on Review tab | Proofing: Set Language | <choose your
language> | uncheck the "Detect language automatically" and "Do
not check spelling or grammar" boxes | Default | OK.
c. Click on Office Button | Word Options | Proofing | check the
"Check spelling as you type" and "Check grammar with spelling"
boxes | uncheck the "Hide spelling errors in this document only"
and "Hide grammar errors in this document only" boxes | OK.
2. If the previous suggestion doesn't help, then in Word 2007
click on Office Button | Word Options | Add-Ins | Manage:
Disabled Items. If you see any disabled items that relate to
spelling, try enabling them.
3. If the previous two suggestions don't help, then:
a. Close Word.
b. Start regedit. In your Windows registry, if the key
"HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing
Tools\1.0\Override" exists, delete it.
c. Restart your computer.
