Instead of using a .xls file, you could save the workbook as an addin
(personal.xla).
If that doesn't work for you, then you could mark the personal.xls file as
ReadOnly (using windows Explorer).
It will make it a little more difficult to make changes to personal.xls, though.
You'll have to remove that readonly mark, make your changes in excel (a single
instance only!), save the workbook, and then mark it as ReadOnly.