Bobh,
Open Task Manager to see if there is a hidden instance of Excel running in the computer's memory. It could be there even if you can't see the actual spreadsheet on the screen.
Press Ctrl + Alt + Del, choose "Start Task Manager", switch to "Processes" and look for EXCEL.EXE. Once found, click on it and press "End Process" in the bottom corner of the window.
Then try over again with MF Pro and your workbook.