When I highlight a word (I using Word 2003) and then type to replace that
word with something else, it should be possible to redo that operation by
highlighting another word and pressing control Y. Recently I have found that
Word regards this as two separate operations: I highlight the word I want to
replace and the first letter I type in replaces it; the rest of the word I
type in is a separate operation. This means that when I press control Y to do
the same thing again somewhere else, I get the new word minus the first
letter.

This is also apparent by looking at the drop-down list of "last actions":
the replacement of the word by one letter is an action and then the typing of
the rest of the word is the next action.

Since I use control Y a lot this is a real pain! What option do I need to
re-set to make the "highlight and type" a single operation? Any help would be
very gratefully received. Thank you very much in advance.