First answer off the top of my head (and there are undoubtedly better ones -
heads and answers) is to use a userform instead of a messagebox and make it
non modal. If the UserForm button click sets a public variable and closes
itself, the main macro  can test that variable. (Don't forget to set it back
when the next instance comes up).

I wonder, however if you could run  the macro in the open  instance of excel  if
there is one so avoiding the readonly box  on reopening personal.xls. This
should happen if you tell the scheduler to open the file (.xls) rather than to
open excel and run the file. Not sure if this helps though.