After focusing on this, I suspect the above scenario could happen to others. So here's a little follow-up info.
If the macro has been activated (from the PERSONAL.xls Workbook), BEFORE opening an EXISTING workbook, it will NOT automatically change the Automatic Calculation of the newly opened Workbook to Manual.
However, AFTER opened, ***ALL OPEN Workbooks*** are at risk of having Calculation set to Manual IF and when the macro is accidentally activated.
*AND*... if the Workbooks are saved with Calculation set to Manual, the Manual setting will be "retained" - until the setting is re-set back to Automatic.
For anyone who needs to know how to check for such a macro in the PERSONAL.xls workbook, here are the steps:
1) From the menu, use: Window - UnHide, and unhide the PERSONAL.xls Workbook.
2) Use <Alt> <F8> (Hold down <Alt> key and hit <F8> key.
This will bring up the "Macro" window.
3) If macros exist, they will appear on the left-side.
4) Click on each of the macros, one-at-a-time, and as you do so, click on "Options" (bottom-right-corner).
5) In the "Macro Options" window, any shortcut key assigned will show under "Shortcut Key".
6) If you want to Delete the Shortcut, use the <Delete> key, and then click the "OK" button.
7) To finish-up, you'll probably want to re-hide the PERSONAL.xls Workbook.
Hope this helps.
Regards, Dale Watson