At the risk of really over simplifying this why not try:

Calling a procedure from the lostfocus method in the text box that holds the key entered by the user. If the key exist, put up a message box saying key already in use and make them re-enter (or display existing data). If the key is not entered yet, then enable the fields.

This way the user can't go past the key text box until you let them and the procedure is a simple seek action.

just my 2cents

Alan