In Word XP or 2003, go to Tools | options | Spelling and Grammar and look at the tick boxes under Grammar. Also look at adjustments that can be made in the Settings button.

Several times when I have done the same thing, either the red wavy line (spelling error) or the green wavy line (grammatical error) appears. It is always worth right clicking the sentence/word to see if there is an mistake. If there is nothing wrong with the sentence/word, the line disappears just as mysteriously.

I can't find it just at the moment, but for Office 2003 there is a Microsoft update on proofing tools - have a look on the Microsoft Update site. This might cure the problem.

I posted just a few seconds too late. However, I think that, in Word 97, the route to the grammar checker is more or less the same.