Thanks to everyone who takes the time to post their problems
and solutions. I can usually find what I need from older
messages, but this time I've been unsuccessful, so here goes...

I'm trying to use

Application.ScreenUpdating = False
Workbooks.Open Filename:= "blah.xls"

to keep the user from seeing "blah.xls" when it is opened.
I also tried

Workbooks.Open Filename:= "blah.xls"
Windows("blah.xls").Visible = False

However, I can still "see" blah.xls on the initial open.
Perhaps I'm just too picky? :)

I've also seen something like:

ThisWorkbook.IsAddin = True on the open event of the called workbook

but I can't just stick a private sub in "blah.xls" because the whole
file is overwritten daily.

I know this can be avoided if I use an XLM macro
(example at http://www.j-walk.com/ss/excel/tips/tip82.htm)
to read data from a closed excel file, but I was wondering if there
was any other way if I use workbooks.open or if I'm using
application.screenupdating improperly. Otherwise, XLM macro it is.
Actually, if I do use an XLM macro, will there be any problems with
portability?

Thank you very much.