Thank you for your post, and I apologize for the late reply.
I already had 10.0v2 installed on my machine, but just to make sure there wasn't a problem with the updater, I removed FileMaker Pro 10 Advanced from my computer (Windows XP SP 3) and installed it again from my CD. I then downloaded the updater and ran it. I opened one of my database files, and I was able to check spelling.
It sounds like FileMaker has lost access to the main dictionary (and probably user dictionary, too). The dictionary should be located:
\Program Files\FileMaker\FileMaker Pro 10 Advanced\Extensions\Dictionaries
Make sure the "usenglish" dictionary is there. If not, then copy this from the CD into this folder.
If you are still having difficulty, uninstall FileMaker Pro 10 Advanced, reinstall from the CD, download a new copy of the updater and run it.
If you continue to have difficulty, please let me know.