Well, I see you have the Google toolbar installed and it has a spell check button, the one with the small letters, ABC beside a check mark. I haven't used the Google toolbar and can't say for sure that it's the source of the behavior you dislike. I found the Google toolbar download page, which lists the features of the toolbar but I see no info indicating whether the spell check feature can be disabled.
If you think the behavior you described is caused by some other program, you could, and may need to, uninstall the Google toolbar to test this theory.
You might try this. Go to Tools, Manage Add-ins, and see whether you can see a separate entry there for spell check, whether from the Google toolbar or otherwise. If so, disable it and see whehter the behavior stops.