I don't know Office 97, but this would work in 2003 and 2007, so it's worth a try:

Go to Tools/Autocorrect options, or the equivalent, assuming this feature was then a part of Word.

In the Autocorrect list, find "cna", or, if you don't see it, type it in the new item box. Word may immediately put "can" in the "Replace with" box, or you can enter it. Then press Delete to delete that correction from the list and return to your document, and test.

Does this resolve your problem?