I have a hidden Excel workbook with code that I put in the XLSTART
folder. When I start up Excel that file opens in the background. I
open up another Excel file that I want the code in the hidden workbook
to run. I then press my macro shortcut keys ("Ctrl-Shift-A" or
whatever) to run the code.

Is it possible to make the hidden workbook check a cell value in each
workbook when the workbook first opens? This way I can look for a
certain value and run the code automatically without pressing any
macro shortcut keys. I cannot put code in the open Excel files
because they are system generated. The code in the hidden workbook is
to format these system generated files. Thanks...